HOW TO REPRODUCE:

With eCommerce and appointment_account_payment installed,
book Mitchell admin on a slot at 9am on a paid appointment.
Once the booking is in cart, book another one at 10pm and with
Mitchell Admin, try to pay the cart. An error message spawns and
warn you that one of the bookings is not available.

ISSUE:

When filtering out unavailable bookings, the availability was
computed on the different time intervals formed by bookings
time ranges. However, the method returning intervals grouped
the contiguous ones. So the user was considered as having a
unique availability on the grouped intervals. Therefore, all
but the first would be considered as unavailable.

FIX:

Use custom means to check all intervals, not merging contiguous
ones. A test is added. At the same time, coverage is maximized
in a simple way for bookings based on users. A condition is also
added for resources when evaluating booking availability to avoid
a crash when a resource is not linked to any appointment anymore.

Task-3570588


for reference, method used for users https://www.geeksforgeeks.org/how-to-implement-interval-scheduling-algorithm-in-python/

This issue occur when the user tries to create an order, and when the order is successfully placed at the point of sale and sent via WhatsApp to receive the order to the user, an error will be generated due to WhatsApp template was deleted.

step to reproduce:
- Install the `WhatsApp-POS` module.
- Open the `WhatsApp`.
- Configuration Menu > WhatsApp Business Accounts > create the account.
- After the complete Configuration of account > delete the `POS Receipt` record.
- Open the `Point Of Sale`.
- Go to Dashboard > Shop (New session).
- Open session > create the one order.
- Enter the number in WhatsApp Receipt > click on WhatsApp symbol.
- The error will be generated.

sentry traceback-
```
ValueError: not enough values to unpack (expected 1, got 0)
  File "odoo/models.py", line 5457, in ensure_one
    _id, = self._ids
ValueError: Expected singleton: whatsapp.template()
  File "odoo/http.py", line 2139, in __call__
    response = request._serve_db()
  File "odoo/http.py", line 1715, in _serve_db
    return service_model.retrying(self._serve_ir_http, self.env)
  File "odoo/service/model.py", line 133, in retrying
    result = func()
  File "odoo/http.py", line 1742, in _serve_ir_http
    response = self.dispatcher.dispatch(rule.endpoint, args)
  File "odoo/http.py", line 1943, in dispatch
    result = self.request.registry['ir.http']._dispatch(endpoint)
  File "addons/website/models/ir_http.py", line 235, in _dispatch
    response = super()._dispatch(endpoint)
  File "odoo/addons/base/models/ir_http.py", line 191, in _dispatch
    result = endpoint(**request.params)
  File "odoo/http.py", line 717, in route_wrapper
    result = endpoint(self, *args, **params_ok)
  File "addons/web/controllers/dataset.py", line 30, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "addons/web/controllers/dataset.py", line 26, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "odoo/api.py", line 466, in call_kw
    result = _call_kw_multi(method, model, args, kwargs)
  File "odoo/api.py", line 453, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "home/odoo/src/enterprise/saas-16.4/whatsapp_pos/models/pos_order.py", line 31, in action_sent_receipt_on_whatsapp
    whatsapp_composer._send_whatsapp_template()
  File "home/odoo/src/enterprise/saas-16.4/whatsapp/wizard/whatsapp_composer.py", line 228, in _send_whatsapp_template
    body = self._get_html_preview_whatsapp(rec=rec)
  File "home/odoo/src/enterprise/saas-16.4/whatsapp/wizard/whatsapp_composer.py", line 281, in _get_html_preview_whatsapp
    return self.wa_template_id._get_formatted_body(variable_values=template_variables_value)
  File "home/odoo/src/enterprise/saas-16.4/whatsapp/models/whatsapp_template.py", line 733, in _get_formatted_body
    self.ensure_one()
  File "odoo/models.py", line 5460, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
```

If the user deletes the WhatsApp template after this commit, it will prevent the
 traceback while sending the receipt through WhatsApp.

sentry-4583290175

Forward-Port-Of: odoo/enterprise#49817

In case the foreign currency is missing on the bank transaction, a partial could be suggested to the user with an exchange difference. This commit allows the user to remove the exchange difference to spare the generation of an exchange difference entry and the creation of a write-off line on the bank reco widget.

Suppose:
1200.0 comp_curr = 2400.0 foreign_curr in 2017 (rate 1:2) 1200.0 comp_curr = 3600.0 foreign_curr in 2016 (rate 1:3) a statement line of 1200 comp_curr and an invoice of 3600.0 foreign_curr.

| Line       | foreign_curr | comp_curr |
| -----------| ------------ | --------- |
| liquidity  |              |    1200.0 |
| new_aml    |      -2400.0 |    -800.0 |
| exch diff  |              |    -400.0 |

...creating a partial reconcile on the invoice.
After removing the partial, you are forced to create a write-off:

| Line       | foreign_curr | comp_curr |
| -----------| ------------ | --------- |
| liquidity  |              |    1200.0 |
| new_aml    |      -3600.0 |   -1200.0 |
| exch diff  |              |    -600.0 |
| write-off  |              |     600.0 |

After this commit, you are able to remove the exchange difference:

| Line       | foreign_curr | comp_curr |
| -----------| ------------ | --------- |
| liquidity  |              |    1200.0 |
| new_aml    |      -3600.0 |   -1200.0 |

...allowing to fully reconcile the invoice without any additional exchange difference entry.

task: 3628473

Forward-Port-Of: odoo/enterprise#52645
Forward-Port-Of: odoo/enterprise#52228