[REF] mail: simplify notification code

Purpose of this commit is to clean notification process: calls, methods
API, method name, variable propagation.

Contains notably

  * simplify API of methods used to group recipients when sending notification
    emails;
  * improve and rename methods used in email notification process;
  * move some methods on model itself as non mail thread records could be
    mass-mailed and _notify_email_headers could be called on other records;

Related to task 1943901
Linked to PR #32404