[IMP] [FIX] mail: mail_thread :
- avoid having to browse the record in message_subscribe: refactored message_subscribe
to work on ids without browsing, try to lessen the number of queries to do
- update _set_followers, method of the function field message_follower_ids, to use
message_subscribe and message_unsibscribe
- in create: add author as follower in creation values, not after create, allowing
create access rules based on followers
- updated message_unsubscribe to be the lower method, not calling _set_followers anymore
[REV] mail: removed some changes to be done in trunk, not in 7.0
[MERGE] Sync with 7.0
[MERGE] [FIX] mail: fixed message_follower_ids function field.

The set method of the field, set_followers, is now using message_subscribe and message_unsubscribe to have only one access point to adding or removing followers. Previously to this fix it was directly creating entries in the mail_followers table, neglecting the calculation of subtypes and default subtypes.