database compaction

Based on the approach laid out in #388 (closed).

WIP:

• Convert everything that needs conversion to Object.normalize().
• Object hot cache
• Verify transparent fetching of remote objects works:
  • Move ActivityPub.fetch_object_from_id() out of ActivityPub module
  • Move id-based containment out of ActivityPub.Transmogrifier.
• Fix object search
• Fix hashtag timelines
• Fix media timelines
• Flatten activity.data["object"] on insert
• Mix task to flatten activity and object database
• Mix task to prune object database
• Tests for all of this