This PR's purpose is to allow certain `div` elements to be handled by the
editor with the features commonly used inside `p` elements.
It is also comprised of a series of fixes and improvements related to
that change, see each commit for more details. Notable changes:
- Fix some behaviors of the DomPlugin `insert` function
- Update how Signature is handled in mail html fields
- Update how qweb fields are handled in the Report editor
- Introduce usage of `div` elements (baseContainer)

Eligible `div` elements will automatically inherit these features as soon as
they met the requirements during the `normalization` phase.

For the user, this means that there are now 2 options for the "base container",
the `div` which does not have a margin-bottom and the `p` which does have one.
`div` will be used as the default in most cases, and `p` will be used as the
default for `knowledge` (and probably `website` when it starts using the
html_editor).

The features around paragraphs are some of the most well defined in the
editor, and with this commit, there are now more ways to fall back on the "right
track" (meaning that user interactions will more likely result in something they
want, rather than not). Most notably, prior to this commit, adding new lines
with `enter` starting from a `div` with one line, then applying an editor
command, would result in the command applying on every line. Now, since the
`div` behaves like a paragraph, adding new lines will create a new editor
context for each line (another `div`), meaning that any command will only apply
to that context.

Technically, this commit introduces a generalization of the concept for
`baseContainer`, meaning that it will be easier to introduce new forms that will
inherit the related editor features.

There are still some limitations:
Some `div` elements will keep their `unsplittable` properties if they contain
HTML elements that are not compatible with a `p` (non-phrasing content), because
any `baseContainer` should be convertible between all equivalent forms.
To avoid such limitations, one solution could be to only work with `div`
elements (and never use paragraph related elements like `p`, `h1`, etc.), which
would be less explicit, but would allow more flexibility.

task-4260398
and
task-4294440