[IMP] website, website_*: improve edit in backend feature

*base, website_blog, website_event, website_forum, website_slides

With this commit:
1. It is now possible to add the 'Edit in backend' entry in the frontend navbar
   for any desired model, not only the ones which have the published mixin.
   It will simply redirect to the main_object form view.
   This commit add it to Forum and Blog.
2. When clicking on 'Edit in backend', it is now possible to land on another
   module than Website.
   That's especially useful for events and slides which are not directly
   related to the website module as they have their own module.
3. Remove the custom Edit in backend from the forum homepage (fa-cog).
   Opportunity was also taken to remove the 'edit welcome message'.
   It will now be editable through the editor (welcome message will appear in
   edit mode). Thus we got rid of the custom edit welcome message controller
   and views.

Closes #36325

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