[FIX] mail: fix auto subscribe subtypes computation

Manual backport of commit done in 13.0 odoo/odoo@9e7092a03f6c44287c661e2b39a943d240aa2bde

This commit contains a followup / rewrite / backport of #34150 :

  * odoo/odoo@16dd3da
  * odoo/odoo@0b11ed8

Fix: propagate generic internal subtypes from parent to childs with auto
subscribe.

  With this commit, when the user subscribes to internal subtypes like
  activities on a parent record (e.g. a project) then the user follows them
  by default on every sub record creation (e.g. all new tasks of this project).

Fix: do not reset existing subtypes when changing parent of a record using
auto subscribe mechanism.

  With this commit, when changing subtype subscription from a parent record
  (e.g. project) avoid resetting all subtypes subscription on existing sub
  records (e.g. all new tasks of this project). Only new records will use the
  parent chosen subtypes. This is done through propagation of followers
  existing policy (see Follower._insert_followers() for more details);

Note that fix done here is slightly different from the one done in odoo/odoo@16dd3da
as there was some mismatch with internal model-related subtypes. Thix fix
will be forward ported and improve the previous one.

Task ID-2205643
Task ID-2241688
COM PR #56775

Co-Authored-By: Thibault Delavallee <tde@odoo.com>
Co-Authored-By: Alexandre Khun <aku@odoo.com>
Co-Authored-By: Priyanka kakadiya <pka@odoo.com>