Notify the user when added as follower by the mean chosen in his/her
preference (inbox/email) and not exclusively by email as before.
Task-2960281
Part-of: odoo/odoo#99371
As mail grows and will continue to grow, ordering views and having right
files is important to understand module organization and content.
Split main.py controller file into two files, one for mail related controllers
(redirections) and one for discuss.
Rename files according to guidelines for wizards and views.
No functional change comes with this commit. This is only code move.
Task-2631873
PR odoo/odoo#75571
Pierre-Yves Dufays