[MERGE][IMP] mail: add a new notification template and use it to notify people of assigned document

Purpose of this merge is
 * to introduce a new notification template much more up to date with
   modern convention about emails;
 * to introduce a new method allowing to push notifications to recipients
   that are not necessarily linked to a document. Those notifications
   can be either Inbox or email according to user existence and
   notification type choice;
 * to make assignation email use this notification mechanism instead of
   an hardcoded email;

Please see commits under this merge for more details about why, what and
how it was done. This merge is related to task 51517. #Closes #22670 .

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