Expose keys in users API

This exposes the list of public keys in the /users/ API method so that
we can consume that data.

This also fixes a bug where the key we stored was just the key...
without the key type or comment.
I previously avoided doing db lookups in the API as this data changes pretty infrequently. Does it make sense to pull this into the graph instead? I'm concerned with the performance implications of adding a db call here where is was previously all in memory.