Jinja as a templating engine was problematic in differents respect:
- introduce external dependency to Odoo (less controll)
- add another templating mechanism in the stack
- specific feature in qweb cannot be reused
- difficulty in rendering easily editable templates
- more knowledge required with no betterment

By replacing jinja with qweb we can now build tools to edit a qweb
that will work with the previously jinja encoded document
(essentially `mail.template` records).

There is a catch however. Some email fields (eg. email_to) used jinja
syntax for rendering dynamic variables (ie. ${object.something} and
${object.something_that_should_not_be_escaped | safe}).

We still want user to use dynamic variables for some char fields (eg.
subject, from, to, ...). We made a new rendering engine called
"inline_template" that will render an expression enclosed by `{{` and
`}}`.

To be able to edit the templates from the backend interface, a
plugin to the Odoo editor has been made for seamlessly edit the
document.

This qweb plugin includes:
- make dynamic variables (eg. `<t t-out="variable"/>`) not editable
  (for preventing the user to shoot himself in the foot)
- group and hide related logical branching (ie. t-if, t-elif, and t-else)
  in order to see only one at once
- a floating select input to switch visibility of a particular logical
  branching

Task-27033

Part-of: odoo/odoo#77074