[FIX] website, *: restore 'animation' system to the way it worked

* website_forum

Following the new editor's merge at https://github.com/odoo/odoo/pull/29775,
the website 'animation' system was 'extended' to vaguely use a
'Registry' instance instead of an object. This was however left
undocumented, with a require in the middle of a file, restarting all
animations when used, only used by website_forum... with more code for
no reason at all, ...

This commit restores the system to the way it was before and re-adapts
website_forum:
- No need to check if website_forum is in the DOM as it is already
  checked by the animation system (on top of that... the new code was
  checking the DOM before it was ready...).
- No need of a dynamic add on a registry, this is always done before the
  DOM is ready, so before the animations are started.
- No need to know the root instance in website forum, just use the
  animation as the Wysiwyg instance's parent.

The goal of this commit is also to avoid useless conflicts with
https://github.com/odoo/odoo/pull/29442 (that should not have had any
conflict at all as totally independent of the editor task...)

closes odoo/odoo#30478