Major database cleanup

[liamwhite]

This addresses a large majority of remaining cleanup items from derpibooru/philomena#28

Remaining from that list after this:

arrays:
  commissions:
    - categories

  filters:
    - hidden_tag_ids
    - spoilered_tag_ids
  
  users:
    - recent_filter_ids
    - watched_tag_ids
    - otp_backup_codes

ActiveRecord workarounds:
  forums:
    - last_post_id
    - last_topic_id

other:
  - convert referencing columns to bigint
  - convert varchar to text

and new things to address at a later date:

unused tables:
  - old_source_changes
  - unread_notifications
  - notifications
  - user_whitelists
  - vpns