make ubuntuone pairing option conditional on service accessibility

We may not have ubuntuone couchdb ready as a replication target for desktopcouch databases, so the module that implements its inclusion in the pairing tool should do some things on startup that test whether all the pieces exist for replication to make sense.

- Look up auth information of user from keyring
- Get its user's db URL prefix.
- Get a list of databases for this user.

If all of those succeed without an error, then claim that the service is "active". If any fail, log it and say the service is not "active".