Postgres document storage backend

[Gastove]

Needed for #24. Several moving pieces to keep track of:

• [x] A documents migration in the DB
• [x] Infrastructure choice: any reason to represent a document as a defrecord, or is a plain Map all we need?
• [x] What the hell is the schema going to look like? (I'll try to write up some notes about this; will comment here once I've got something.)

[Gastove]

Design doc here

[Gastove]

Implementation note to point out: the very easiest way to provide search for HTML docs is to serve those HTML docs from the DB. But, there's a whole mob of docs that go in to making the HTML docs work -- for instance, CSS files. We can either:

1. Only put HTML in the DB
2. Only index HTML docs for search

Right now, I'm implementing no. 2, because 1 sounds like way to big a pain in the ass. But: I'm open to it, if somebody can think of a really compelling reason why we need it.

[Gastove]

Done!