Support authentication against GDS Signon
Redirect login and logout pages to Signon
Fix tests to reflect new auth flow
Add missin acceptance specs for existing user flow
Implement flag to remotely sign out user.

This only implements the auth flow part of this, forcing a re-auth if
the flag is set.  Implementing the callback endpoint to actually set
this flag will follow later.
Add an index to the User.uid field

Given sso integration looks users up by this field, it's a good idea.