### Description of the issue/feature this PR addresses:
When calling `/shop` route (or any similar one), big files (for eg. image_1920) are read while they shouldn't.
The render of shop route triggers the method `_get_combination_info` and this method is reading all the big files just to know if they are there.

The problem is big if for those files you have some kind of latency (for eg. nfs storage or using https://github.com/camptocamp/odoo-cloud-platform/tree/13.0/attachment_s3)

I'm not sure which would be the proper way to fix this. (clearly not as done on this PR)

This is the how methods are called 
```

  /home/odoo/custom/odoo/addons/website/models/ir_ui_view.py(344)render()
-> return super(View, self).render(values, engine=engine, minimal_qcontext=minimal_qcontext)
  /home/odoo/custom/odoo/addons/web_editor/models/ir_ui_view.py(27)render()
-> return super(IrUiView, self).render(values=values, engine=engine, minimal_qcontext=minimal_qcontext)
  /home/odoo/custom/odoo/odoo/addons/base/models/ir_ui_view.py(1199)render()
-> return self.env[engine].render(self.id, qcontext)
  /home/odoo/custom/odoo/odoo/addons/base/models/ir_qweb.py(58)render()
-> result = super(IrQWeb, self).render(id_or_xml_id, values=values, **context)
  /home/odoo/custom/odoo/odoo/addons/base/models/qweb.py(260)render()
-> self.compile(template, options)(self, body.append, values or {})
  /home/odoo/custom/odoo/odoo/addons/base/models/qweb.py(333)_compiled_fn()
-> return compiled(self, append, new, options, log)
  <template>(1)template_website_sale_products_52()
  <template>(2)body_call_content_50()
  <template>(88)foreach_46()
  <template>(89)foreach_45()
  /home/odoo/custom/odoo/odoo/addons/base/models/qweb.py(333)_compiled_fn()
-> return compiled(self, append, new, options, log)
  <template>(1)template_website_sale_products_item_3018()
  /home/odoo/custom/odoo/addons/website_sale_stock/models/product_template.py(22)_get_combination_info()
-> parent_combination=parent_combination, only_template=only_template)
  /home/odoo/custom/odoo/addons/website_sale/models/product.py(286)_get_combination_info()
-> parent_combination=parent_combination, only_template=only_template)
  /home/odoo/custom/odoo/addons/sale/models/product_template.py(246)_get_combination_info()
-> display_image = bool(product_template.image_1920)
  /home/odoo/custom/odoo/odoo/fields.py(1045)__get__()
-> recs._fetch_field(self)
  /home/odoo/custom/odoo/odoo/models.py(3002)_fetch_field()
-> self._read(fnames)
  /home/odoo/custom/odoo/odoo/models.py(3102)_read()
-> field.read(fetched)
  /home/odoo/custom/odoo/odoo/fields.py(2013)read()
-> for att in records.env['ir.attachment'].sudo().search(domain)}
  /home/odoo/custom/odoo/odoo/fields.py(2013)<dictcomp>()
-> for att in records.env['ir.attachment'].sudo().search(domain)}
  /home/odoo/custom/odoo/odoo/fields.py(1062)__get__()
-> self.compute_value(recs)
  /home/odoo/custom/odoo/odoo/fields.py(1981)compute_value()
-> super().compute_value(records_no_bin_size)
  /home/odoo/custom/odoo/odoo/fields.py(1147)compute_value()
-> records._compute_field_value(self)
  /home/odoo/custom/odoo/odoo/models.py(4018)_compute_field_value()
-> getattr(self, field.compute)()
  /home/odoo/custom/odoo/odoo/addons/base/models/ir_attachment.py(199)_compute_datas()
-> attach.datas = self._file_read(attach.store_fname, bin_size)
```



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Forward-Port-Of: odoo/odoo#83181
Forward-Port-Of: odoo/odoo#81767

- The align-self and justify-content styles don't work on cells so when converting a Bootstrap column to a table cell, we should also apply a vertical alignment. The same applies to align-items on rows.
- The conversion from rem to px was rounding the value to 1 decimal place but the browser handles up to 2 decimal places. As a result there were visible inconsistencies in the conversion (12.25px in the original becoming 12.3px in the converted email).
- When converting from grid to tables, we need to convert values from x/12 to x/100. The rounding error was a little too big, making it too visible at times. With this commit we now round to 2 decimal places instead of rounding to the nearest integer.
- When fonts with a round border were converted to images, the dimensions often ended up slightly off, and most visibly a little bit cropped by the border. Note that this also removes the "alpha" argument of the font_to_img route since it wasn't used anywhere and transparency is not supported in emails anyway.
- The mail-safe font is applied to a style in <head> for emails. But the way it was applied, <div>s were forgotten. Since most of those are converted to tables when converting body_arch to body_html, it resulted in visible font differences between body_arch and body_html.
- When converting, we ensure each table has a tbody but inadvertently were wrapping each <tr> in a separate tbody rather than wrapping all the contents of the table.
- Cards need to be double wrapped in tables to be displayed properly because of a bizarre hack by bootstrap that uses background-color rather to give the effect of a border, and because we can have card-body be a sibling of a list-group. Incidentally this also prevents the introduction of a new row for each whitespace text node.
- The conversion of padding from grids to their equivalent tables was sometimes faulty due to an error in the evaluation of the cell's position within the table.
- Images sometimes shrunk on conversion because of a parenthesis error in a conditional chain, because dimensions set via element attributes should be preserved, and because the offset height should be used rather than the element height as returned by jQuery.
- The conversion process of emails for mail clients involves retrieving the dimensions of images and icons in order set them as element attributes. This can however not possibly work if they are invisible at the time of conversion. This therefore makes sure they are visible by changing the visibility of whichever parent was invisible before conversion and restoring it when we're done.

Enterprise PR: https://github.com/odoo/enterprise/pull/23249

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Forward-Port-Of: odoo/odoo#83164
Forward-Port-Of: odoo/odoo#82233