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

Guidelines: re-organize model files and split them according to main
models. Also reorganize views.

Move some code bits / fields declarations to reorder them. Purpose is to
make code easier to understand and find. Funny we found two override of create
that have been merged. An override of name create is not necessary considering
code was present in one of the two merged create. Make internal methods private
and rename send_mail to action_send_mail to avoid confusion with composer and
template send_mail methods.

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;

Rename mass mailing models. Rationale :

  * remove mail prefix, as mass mailing will allow to use SMS to contact
    people and will soon be less mail-dependent;
  * have simpler name, easier to read, find and understand;
  * mailing.list/contact/tag seems a better naming than mail.mass_mailing.
    list/contact/tag;
  * have a mailing prefix for all models used in mass mailing;

MIGRATION

mail.mail.statistics model -> mailing.trace
mail.statistics.report model -> mail.trace.report
mail.mass_mailing.list model -> mailing.list
mail.mass_mailing.list.merge model -> mailing.list.merge
mail.mass_mailing.contact model -> mailing.contact
mail.mass_mailing.list_contact_rel model -> mailing.contact.subscription
mail_mass_mailing_contact_list_rel table -> mailing_contact_list_rel (specific
case of a decorated m2m)
mail.mass_mailing.stage model -> mailing.stage
mail.mass_mailing.tag model -> mailing.tag
mail.mass_mailing model -> mailing.mailing

fields updated (w column change)
  * link.tracker.click: mail_stat_id -> mailing_trace_id

fields updated (no column change)
  * mail.mail: statistics_ids -> mailing_trace_ids
  * mail.mass_mailing (mailing.mailing): statistics_ids -> mailing_trace_ids
  * mail.mass_mailing.list (mailing.list): subscription_contact_ids ->
    subscription_ids

LINKS

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