(Bug 5254) Fallback to ext_123.livejournal.com

For non-openid-based identity users on LJ, we won't be able to find a suitable
identity URL. So just use ext-1234.livejournal.com to identify them.

(Note: ughhhh I can't test as a whole system, because I don't have an
account to import that has the problem. But I have tested the function
that remaps the remote user to a local user)