WIP: Cleanup of non-pipeline inserts/updates

• Removes unused functions from Transmogrifier
• Remove Pleroma.Web.ActivityPub.ActivityPub.insert()
• Keeps support for Listen activities ⇒ !3487
• Keeps support for Page objects ⇒ !3097 (merged)
• Create an Ecto validation for tag field (probably similar to attachments) and move Transmogrifier.fix_{tag,emoji,…} into it ⇒ !3244 (merged)

Separated:

• Unfollow: !3486

Depends on:

• !2984 (merged) (note)
• !3097 (merged) (page)
• !3203 (merged) (AP C2S)
• !3487 (listen)