*: auth_signup, knowledge, mass_mailing, portal, survey, test_website,
   test_website_modules, test_website_slides_full, web, web_editor,
   web_tour, web_unsplash, website_blog, website_crm,
   website_crm_partner_assign, website_customer, website_event,
   website_event_exhibitor, website_event_meet, website_event_track,
   website_forum, website_hr_recruitment, website_knowledge,
   website_links, website_livechat, website_mail_group,
   website_mass_mailing, website_membership, website_sale,
   website_sale_autocomplete, website_sale_comparison,
   website_sale_loyalty, website_sale_slides, website_sale_wishlist,
   website_slides, website_slides_forum, website_twitter, base

A new client action is introduced, to display the website in an iframe.
This allows to remove the "frontend" Odoo UI entirely. Everything
related to the website configuration (and especially the edit mode) will
now be done on the backend side.

The idea behind this mainly comes from the fact that we needed the
website in an iframe during edition to be able to render a mobile
preview properly. It was also required so that options which reloads
the page... are able to only reload the page and not the entire editor
with it. From there, it was decided that actually, since the website is
in an iframe, it could be in the backend and every website UI could
rely on this.

In the end, this comes with many functional and technical advantages.
Here are some of them (a few of them are not entirely done with this
merge but will follow the merge in the upcoming weeks, see this as a
non-exhaustive list of possibilities that are unlocked by this work):

Technical:

- No duplicated logic for website UI and backend (purple navbar is the
  backend one, slightly customized).
- No conflicting logic for website UI and website content (like dialog
  opening, ...).
- No forced CSS for our website / web_editor UI, no conflict with
  website themes.
- Everything converted in OWL, in backend code (no special OWL for
  frontend).
- "Legacy" code moved as frontend code or removed.
- No website content CSS for the case "when there is the Odoo purple
  navbar at the top".
- Backend views & features available (e.g. creating a new event -> form
  view instead of custom code with onchanges, validations, etc)

Functional:

- All website features available while editing your website (product
  configuration, settings, etc), no switch backend/frontend, no
  redirection, no reload (just the iframe).
- All other app features available while editing your website (discuss,
  calendar, all other apps, ...).
- Typing a website URL in your browser reaches your website (very small
  Odoo UI overlay at the top left as a connected user).
- Mobile preview: directly resizing the website rendering, in edit mode
  too and with a better phone image.
- Editor: mobile view edition (the editor is instanciated around the
  website content instead of inside of it (iframe))
- Can reload the website without reloading all the tools (editor, html
  editor, menu edition, etc) (customize_show options for instance ->
  moved as standard editor options which reloads the website only)
- Website UI in your language instead of website language

On the backend side, some customizations were done so that the UI feels
like it works as before (note that it will be reworked in upcoming weeks):

- A `website_systray` registry is defined to replace the systray items
  with the website's one when previewing the website.
- The webclient's navbar is patched to display additional dropdown menus
  related to the website edition. 
- A website service that holds the global state of the website is
  introduced. Components from the systray and the dropdown menus can
  access and modify it.
- A component adapter for the wysiwyg (that will display the snippets
  menu) is added, that allows for caching the wysiwyg libraries, thus
  improving editions responsiveness.

See sub-commits for details. This work is known to not be bug-free and
will be bug-fixed, improved and reworked in the upcoming weeks.

task-2687506

Related: https://github.com/odoo/enterprise/pull/28448
Related: https://github.com/odoo/design-themes/pull/572
Related: https://github.com/odoo/upgrade/pull/3464

Thanks to the whole team for reviewing and testing !
(ARD, BVR, BSO, GDI, MOU, PDE, QSM, RDE, SOBO, YOL)

Co-authored-by: Younn Olivier <yol@odoo.com>
Co-authored-by: Arthur Detroux <ard@odoo.com>
Co-authored-by: Mehdi Outagant <mou@odoo.com>
Co-authored-by: Benjamin Vray <bvr@odoo.com>