apps/activities: prepare removal of activities app

[goapunk]

• migration to delete models
• delete all files which can already be safely removed

partly fixes #3600

Tasks

• [ ] PR name contains story or task reference
• [ ] Documentation (docs and inline)
• [ ] Tests (including n+1 and django_assert_num_queries where applicable)
• [x] Changelog

[goapunk]

Need to remove the app from the INSTALLED_APPS too. And do we include it in the coming release?

We can include it now I'd say and once it was deployed to prod we can remove it from the INSTALLED_APPS and remove any leftovers