Site login uses FirstOrDefault account in DB instead of SingleOrDefault

Prevents http://zero-k.info/Forum/Thread/7364#98980

It might be better to just prevent duplicate names in DB to begin with, but this might cause problems of its own if uberserver and ZK DBs desync for whatever reason...
This is a symptom. Something in the code causes duplicate creation of accounts. These have to be merged from time to time .. http://zero-k.info/users/duplicates
Here its extra dangerous because association between lobby id and zk account happens here. Not sure if its right to change.