Main issue:

Prior to this commit, updateValue would guarantee that `body_arch` and
`body_html` were always updated in sync (to avoid an inconsistent state where
the user thinks they updated the mailing, and they see the new html in the
editor, but the email html is obsolete and would be sent as is).

However this caused another issue, that the user would lose their work when
switching tab in the Notebook, because computing the `body_html` may be slower
than the view patch to switch tab (`updateValue` is interrupted). In such a
case, neither `body_arch` nor `body_html` was updated on the record, and when
the user comes back to edit them, they see that all changes were lost.

To prevent this, a new strategy is adopted:
- `body_arch` is now updated as soon as possible to save the latest user
  changes
- `body_html` is set to an empty string to avoid inconsistencies, and in order
  to trigger `convert_inline` automatically the next time the
  `mass_mailing_html_field` is instanced without any user action.
- after `convert_inline`, if `updateValue` was not aborted, the record is
  updated again with the new value for `body_html`

Minor issues:

1) Prior to this commit, the iframe would flicker during `convert_inline`
because it was done inside that very iframe, and the `convert_inline` needs a
specific width (1320px) for it to work properly, meaning that if the iframe was
not at that specific dimension, it was resized temporarily for that process.
This commit introduces hooks to execute the `convert_inline` process in a
separate iframe, outside of the user view, which removes this flickering.

2) Prior to this commit, assets for readonly/basic editor/builder were all
loaded inside the iframe, and then toggled on/off depending on which were
needed. Since the `convert_inline` process is moved in another iframe, it's as
good time as any to deprecate this toggling (as it could be a bit unreliable and
could cause some flickering when unloading/reloading the style). There is now a
separate asset bundle for the 3 use case, and only one of them is loaded per
iframe depending on the needs.

3) Prior to this commit, `body_html` was hard-coded as a dependency of the
`mass_mailing_html_field`, and that dependency lacked the `required` attribute,
which should depend on the value of `body_arch`. The dependency is now added in
the related views, and the field is now generic. This also prevents the user
from leaving the view if `convert_inline` could not be completed successfully.

4) Prior to this commit, switching to another view through `doAction` could
throw an error if the field was dirty, as the `form view` would be destroyed
before `commitChanges` had the time to be completed. Now, `commitChanges`
promise is properly awaited by the `action_service` if the field is dirty before
`doAction` (unless `forceLeave` is true).

5) Prior to this commit, the record was updated through a `blur` event on the
iframe window. However, it means that every time the iframe looses focus when
the users interacts with the builder, a popover or the form status indicator,
that blur event would fire, triggering the `convert_inline` process. This commit
reduces the amount of such updates by only triggering the update outside of
these elements, as we don't need to update the record value while the user is
still actively editing the mailing.
Blur handlers/props are deprecated with this commit and will be removed further
down the line.

6) Ensure that in the rare case where a `html_field` value is exactly the same
on different records, the `html_field` state key is updated (triggers a
wysiwyg/mass_mailing_iframe reset).

7) Remove an erroneous part in `onWillUpdateProps` of `mass_mailing_html_field`
which could display the theme selector again on the same record just after
selecting a theme if props were updated.

8) Ensure `withBuilder` getter of `mass_mailing_html_field` properly reads the
state activeTheme every time it is used (if it does not, it could cause issues
with the reactivity, since reading on the state is required for a property
subscription).

9) Remove a useless `onWillUpdateProps` of `mass_mailing_iframe` which was never
used because when `props.showCodeView` changes, the iframe is always destroyed,
so there is no need to update its state.

10) Deprecate usage of `<meta http-equiv="X-UA-Compatible" content="IE=edge"/>`,
to be removed further down the line, as it is useless in the modern web.

11) Ignore errors during a builder "Operation" that was not finished before the
editor was destroyed. In `mass_mailing`, the editor is expected to be
destroyable synchronously to work inside an Odoo view, unlike in `website`.
What's already in the DOM just before destruction will be updated on the
`record`, and the rest of the operation will be lost. However, an attempt is
made to wait for ongoing operations at the start of the
`HtmlField.commitChanges`.

task-5976348