[MERGE][IMP] mass_mailing: allow to choose outgoing server for mailings

This merge improves mass mailing application by allowing to choose
specific outgoing mail servers on mailings. It is used notably if
you want to separate mass mailings from classic emails and/or to perform
some load balancing. A settings to add a default server used for mailings
is also added.

As a read access on outgoing mail servers is added for mass mailing
users the credentials are now restriected to admins for security
reasons.

This merge is related to task ID 31737. Closes #21699 .