Replace postgres.py with SQLAlchemy

[patricksnape]

This will make it much much simpler to support many database backends. It also makes martialling the returned objects much more simple. We would have to play a bit with the default table creation depending on of the json type exists or not, but that shouldn't be too hard (full back to just a blob and stringify the json).

[jabooth]

True this would make it much nicer to maintain, especially if we expand out what we are storing in the DB a little.

My worry with an ORM is that I really like the simplicity of what we have now - I can manually inspect the schema with a DB browser like Postico, make a few changes directly, and know I'm not screwing anything up. Would we have the same level of robustness with SQLAlchemy? (I'm asking as I really don't know btw, not rhetorical!)