[FIX] mail: allow editing of mail mail

As it holds full HTML documents, mail.mail often encounters issues with
the html editor.

We piggy-back off the intoduction of the switching
behavior between code-view and readonly-view behavior introduced in
a5c0085ce91570c6c961e3293dfe50f4faba6043

The 'sandboxed preview' mode can now be enabled with an option
and will ensure all content can only be viewed in a sandboxed iframe
or in the code editor.

This re-introduces editing, which was mistakenly fully disabled in the
usability fix that introduced body_content here:
5055b374

Some tests are added to ensure the key points of this commit remain
the same. Tests are re-ordered alphabetically to match the order
of execution, making debugging easier.

task-3255777

closes odoo/odoo#118423

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>