Check for renamed user on login

If a user logs in with the pre-finalization username, but their old
name doesn't exist as a global account anymore (winner renamed
themselves), then show a message on the login page with the user's
new name.

Relies on core change If48d59afa63ace68c147eca952f1d4f43acc105f

Bug: 67995
Change-Id: I06b9b6322e408868f516aeabd61c6580f304e009