Working protoype of full text search (Postgres-only)

Refs #15.

This needs more work:

* [ ] make updating based on RQ
* [ ] write a sensible UI for the search form (maybe a dropdown like the navigation)
* [ ] make the search form show up everywhere
* [ ] think about whether using the raw search abilities would be better
* [ ] fix the all/unread links to carry over the search query