This is a cleaning pass after the `mass_mailing` [refactoring].

### Stop using `web.assets_frontend` for mailings

For multiple reasons:
- loading this bundle implied that all frontend JS and SCSS were loaded for a
  mailing, while the mailing actually only uses css, and only a fraction of it.
- this bundle is crowded with assets from all modules that are totally
  irrelevant to `mass_mailing`. i.e. not loading it prevents issues where
  `website` (or any other module) style assets modified `mass_mailing` assets in
  a way that was not desirable.

This will drastically improve loading time when starting a new mailing for the
first time, and will also speed up the `convert_inline` process since a lot less
style rules need to be parsed.

This implies that the `snippets preview dialog` can no longer load
`web.assets_frontend` by default. Instead a resource was introduced for each
module to introduce their own previewing assets through a plugin.

### Improve UX for an empty mailing

Ensure that a user typing directly in the `o_editable` element will not result
in a corrupted content, and that the UX for an empty email hints at the user
what to do.

This results in a better experience when a user uses the `empty theme` for a new
mailing.

### Fix snippets
- clean snippets options (remove duplicates, redundant, ineffective
options).
- ensure that no editable phrasing content sits next to flow content (which is
  not well supported in the editor).
- ensure that style assets for snippets correctly use the `mass_mailing` bundle.
- ensure that snippets use the proper technical classes.
- ensure that when `col-x` is specified, `col-md-y` is specified too, to avoid
  bumps in the interface when the user tries to redimension columns.

[refactoring]: https://github.com/odoo/odoo/commit/82969dc5c6c36a7a91194cf15b33c9abb560a8e7

task-5078825

Co-authored-by: Damien Abeloos <abd@odoo.com>
Co-authored-by: Thomas Josse <thjo@odoo.com>

When changing the expense policy of an expense product, the
compute of analytic distribution of all expenses linked to the product
is triggered.

Steps:

- Have an expense product X with expense policy 'at_sales'
- Create several expenses with an expense product X and  any analytic account
- Create an analytic distribution model that link the expense account of
  X with a specific analytic account AA
- Create a new expense for product X, the analytic account AA should be set
  from the distribution model
- Go to the form view of product x and change the expense policy to
  'cost'
- Go back to the expense list view
-> All expenses having the product X have the AA account

Cause:

`sale_order_id` has been added to the `depends` of `hr_expense._compute_analytic_distribution`
by 2b3bf5e0fe31d4b4ef6b487da493657f695b14e1 but this wrong since we have
the `sale_expense._onchange_sale_order_id` that add the
`analytic_dostribution` field to the fields to be computed.
The compute is triggered since we change `product_id.expense_policy`,
which triggers the `_compute_can_be_reinvoiced` which triggers
the `_compute_sale_order_id`

Fix:

With this commit, we emove the depends on the compute and we also adapt
`test_compute_analytic_distribution_expense` in a way that it triggers the onchange
as we do in the original flow.

opw-4998899

Forward-Port-Of: odoo/odoo#226864
Forward-Port-Of: odoo/odoo#224226