[MERGE][REF] mail: optimize message notification process

This merge optimizes and lessens query count of notification process. When
posting a message some computation is done to fetch followers and/or people
and channels to notify, then notify them on their Inbox or by email. This
process now takes less queries thanks to several improvements :

 * fetching followers / recipients data is now done manually in SQL allowing
   to fetch data across several models in less queries;
 * computing recipients is now separated from notifying them. Notification
   better uses precomputed data in order to avoid fetching and browsing
   several time the same data;

When possible code has been made more readable through code inlining. Several
methods have been removed because they are not used in inheritance. Some code
is now more low-level. Optimizing part of the process can make it harder to
inherit or modify.

Performance counters are updated showing gain in nearly all tested cases.
An enterprise branch is linked to this one that also improve notification
process.

This merge is linked to task ID 47934 and closes PR #24033. No functional
change should occur with this merge.

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