[IMP] web_editor, *: review the editor design CSS

*: web, mass_mailing, website, website_form, website_sale

New CSS design of the editor (step 1/x).

This work is unfortunately far from perfect but is a first step towards
what we want. In particular, much CSS linked to elements that are meant
to be removed/moved in the upcoming weeks was done in a huge rush but
will thus hopefully be removed/reviewed before the 14.0 release.

As the design is now far more complex including gradients and other
"complex" elements, it was also very difficult to keep a design based on
css-variable so this system was removed, which means that the editor now
appears the same everywhere (backend, frontend, ...). The original goal
of this system was to have an adapted style for backend, frontend and
website visitors. Unfortunately, the current result was a very good
frontend editor design, a not-so-good backend editor design (for mass
mailing at least) and a totally unadapted design for the website
visitors. Making this perfect will be achievable by loading different
assets for those 3 areas and hopefully those assets will only differ by
the values of editor scss variables.

Part of https://github.com/odoo/odoo/pull/53930
task-2157252