(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.
Merge pull request #601 from afuna/importer-bug5254

(Bug 5254) Fallback to ext_123.livejournal.com