[REF] website_links: set is_website to True on the 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 changes website_links.js so that, when creating a campaign,
the field is_website is set to true.

This will allow us to filter irrelevant campaigns in mass_mailing and social

Task ID: 2002029
PR: #34015