The get_or_create function is vulnerable to race conditions in MySQL, which can
cause the model LoginFailure to, in some cases, have more than one row for the
same user, breaking the login for that user.
Addinf functionality to expect and clean the error by deleting extra rows (by
oldest lockout date), leaving just one entry and allowing the user to login.
Replayed and squashed by @efischer19, initially commited by @laq
