[REF] utm,mass_mailing: replace mass mailing campaign by utm campaign

PURPOSE

This commit removes the mass_mailing.campaign model. Instead of having a fully
fledged model, we will simply inherit utm.campaign. We will also add relevant
statistics on utm campaign model in order to use it in various applications.

SPECIFICATIONS

This commit removes the mass_mailing.campaign model. Instead of having a fully
fledged model, we will simply inherit utm.campaign. This change implies that
mass_mailing.tag and mass_mailing.stage have to move to the utm model along
their associated views/data.

These changes were made so that campaigns could be used in the future
by social, mass_mailing and mass_sms and available in the same view

This commit also removes the source_id and the medium_id
fields on the campaign.

This commit also moves the unique_ab_testing field from the mass_mailing_campaign
to the mass_mailing model

Task ID: 2002029
PR: #34015