Feature: actually working models

This is a fairly catchall PR that:

1) Simplifies the models down to Artist, Master, Release, and MusicUpload (see #15). Upload and UploadGroup were duplicates of Release and Master, respectively, and were removed - it may be useful to add them back in if we have the luxury of supporting arbitrary upload types, but for now they overcomplicate things.
2) Resets Model migrations for the music app.
3) Kills off the Discogs API proxy. This should have been part of a second PR, but I didn't notice I'd done so until I reviewed my changelog.
4) Updates tests for added/removed features.