[REF] mail: improve implementation of auto subscription

This commit refactors mail.thread message_auto_subscribe method. Purpose
of this method is to automatically subscribe people to documents in several
scenarios

 * followers from an umbrella record to sub-records, like followers of project
   added to tasks;
 * responsible of documents, in addition to sending notifications to newly
   added responsibles;
 * some specific use case depending on some applications like managers
   for expense reports;

New auto subscription now uses the recently introduced method optimized to
compute new or to-update followers. It is also rewritten so that overriding
it is easier. Indeed simply overriding _message_auto_subscribe_followers is
now sufficient to add new followers to a record, depending on some updated
values. It is also now possible to specify a template to use for some
specific notification, if requested.

Fetching auto-subscription data in subtype is also optimized. It is now done
in a single SQL query. All data related to parent (like project) and child
(like task) as well as default and internal-only data is prepared in a single
query and given back to auto subscription process. This allows to gain
queries in auto subscription based on subtype like project / task use case.

This commit has a great impact on performance counters. Complex use cases
involving tracking and responsible notifications gain about 30 queries which
is about 15% of their total query count.

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