[FIX] mail: backport of saas-2 revision 9065 and 9081.

Make sure the user is subscribed before the super() call in mail_thread. This allows to have follower based record rules working (eg: note.note)
[IMP] mail: check at least access_rights in case of creation
[IMP] mail: in case of creation, still check creation access rights for subscription
[MERGE] [FIX] mail: mail: backport of saas-2 revision 9065 and 9081 and security improvements.

Add followers before super call in mail_thread creation.
This allows to have a value for the _set_followers call and be able to have record rules based on followers at creation.

Set the priority of message_follower_ids fields to -10 (default is 0) to be sure it will be executed before other function fields using these followers (eg: _set_stage_per_user from note.note does a browse which would trigger record rules to read the note)

Context hack to avoid checking read access at creation of the record (check create rules instead)