www: Migrate userdb to cockroachdb

[lukebp]

We currently use LocalDB to store user data. LocalDB does not support concurrent connections. Part of scaling politeiawww is being able to run multiple politeiawww instances. In order to do this we need to use a database that supports concurrent connections. We use CockroachDB for everything else so it makes sense to migrate the userdb over to CockroachDB.

#689 took a crack at this a while back, but a lot of things have changed since then.

When we migrate the user data over to CockroachDB we also need to change how it is stored. The public user fields (userID, username, pukeys) should be queryable fields and everything else should be stored as an encrypted blob.