Schema changes to aus

Update the schema a little bit, and add some functionality to content providers and archival units:

 * Add a foreign key from Aus to ContentProviders, necessary for finding an open AU for a deposit, among other things.
 * Make contentProvider extract the hostname from the permission URL.
 * Generate uuids if they aren't provided.
 * Get new AUs from ContentProviders, get the open AU from a content provider (or create one if there isn't already an open AU).

All with tests, and all passing travis.