[FIX] mail: subscribe user at creation as a default value instead of post processing.
This allows to have a value for the _set_followers call and be able to have record rules based on followers (eg: note.note).
Set the priority of message_follower_ids fields to -10 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)