[FIX] mass_mailing: fix management and propagation of UTMs

PURPOSE

Mass mailing is currently a bit messy. As SMS will be added as a way to notify
people in mass through SMS let us take this opportunity to somehow clean this
application: organization, light code cleaning, model renaming.

SPECIFICATIONS

Fix UTM management and propagation in mass mailing.

Right UTM definition in mass mailing

  * mass mailing campaign -> utm.campaign
  * mass mailing -> utm.source
  * "email" -> utm.medium

Therefore

  * remove campaign setting source and medium as each mailing is a source
    and medium is "email";
  * ensure each mailing is a separate source (otherwise name is shared);
  * ensure UTM values propagate to link creation are those values and not
    the one coming from the mass mailing campaign;

LINKS

Task ID 2037906
Preparing task ID 1997464 (SMS addition in mass mailing)
PR #34938

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