Fix foreign keys in user_messages

Not sure why the other syntax wasn't creating them. Although, this does make
the table a little larger... the user_messages -> messages foreign requires
another index. Blergh.
Add additional fields and indexes

Base strings for class/instance, also conversation, also a bunch of indexes.
Also a migration script. Issue #11.
Simplify addUserSubscriptions

If creds were supplied, use them whether or not there were any private subs. We
don't really need the POST /v1/zephyrcreds API.