@wip Migrate from MongoDB to Postgres

I've rebased Undev@d51612ff7be776d7dfe899254768d4f059b72654 onto master. (c.f. #614)

I squashed @realmyst's commits for two reasons:
 1. many commits left the codebase in a transitional state
 2. to expedite rebasing

I'm now working through failing specs.

I'm submitting this WIP pull request to show my work and to starting getting feedback on the changes.

Note: I plan on force-pushing to this branch while I work, primarily to trim the diff down to only those changes that pertain to Postgres.