[FIX] website: ensure only editor can force website with URL 'fw'

Before this commit, everyone could use the `fw` query string to force a
website during the dispatching. Plus, it would not require multi-website to be
enabled (multi-website only display UI elements, it does not affect behaviors
as they are already triggered even if multi-website is disabled).

This was intended but caused an issue for public users.
Indeed, the code that would force a website during the http dispatching would
be executed in `_add_dispatch_parameters`, after the auth methods.

Then, the request.uid would be the one from the website we were leaving and not
the one of the website we are forcing.
Public user would then not be recognized as such (if both websites have their
own public user) and would have interface glitchs (considered as connected).
Note: Next reload or page navigation would set the request.uid correctly.

Now:
To avoid such a behavior, we decided to allow only designer user to be able to
switch website with the `fw` query string, and only if multi-website is
enabled.
Note: The designer group check is done on the leaving website and not the
      forcing one as `_add_dispatch_parameters` is executed after auth methods.
      There is no way to check with which user the user will be logged on the
      forcing website (Public, Same user, different user?).
      Also, it would have to handle the same domain/different domain.

Step to reproduce:
  - Activate multi-website
  - Activate multi-company
  - Create SiteA and SiteB
  - Create CompanyA and CompanyB
  - Set CompanyA on SiteA and CompanyB on SiteB
  - A new public user will be bootstraped for CompanyB
  - Optionnaly, setting manually a different public user for CompanyB will do
    the same.
  - Go to website 1, request.uid is PubUserA and request.website is SiteA
  - Navigate to website 2 with `?fw=2` in URL, request.uid will be PubUserA and
    request.website will be SiteB.
    This is wrong, request.uid should be PubUserB as we are on SiteB

This will result in multiple bug, mainly the non logged user will see the
dropdown in the navbar as if they were logged in showing 'PubUserA'.

Fixes #27998

closes odoo/odoo#29136

addons/website/models/ir_http.py

@@ -74,7 +74,9 @@ class Http(models.AbstractModel):
 
         # Force website with query string paramater, typically set from website selector in frontend navbar
         force_website_id = request.httprequest.args.get('fw')
-        if force_website_id and request.session.get('force_website_id') != force_website_id:
+        if (force_website_id and request.session.get('force_website_id') != force_website_id and
+                request.env.user.has_group('website.group_multi_website') and
+                request.env.user.has_group('website.group_website_publisher')):
             request.env['website']._force_website(request.httprequest.args.get('fw'))
 
         context = {}