[IMP] web_editor, *: introduce and restore new web_editor UI

* mass_mailing, note, website, website_blog, website_form,
website_mass_mailing, website_sale

This commit, unfortunately, mixes three things:

- Restoring as much as possible the scss organisation to allow styling
the web_editor UI properly.

- Fixing some bugs like a border around the page once the editor is
loaded, no ability to scroll the snippets, etc

- Introducing a whole new UI for snippet options: a left panel instead
of the old dropdown & button overlay.

Note: this commit also do some linting and ES6 convertion even though
some of it has been done in the parent commit.

Note 2: some elements that were removed are still styled in the POS apps
but this is because part of a feature was removed while leaving dead
code behind, this is handled in another PR which is to be merged
(https://github.com/odoo/odoo/pull/36136).

Part of https://github.com/odoo/odoo/pull/36068


task-1942370

closes odoo/odoo#36068

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>