allow client to discover details of server-side login provider
allow multiple UIs for providing login credentials

Current implementation requiring user to enter username and
password is the only one supported currently.

Next step is to allow login via a "preauthorized" username,
e.g., when login authorization comes from a trusted
proxy server.
added a login view for "preauthorized user" logins
bugfix to RemoteUserLoginProvider.getPreAuthorizedUsername()

Was trying to get the value of X-Remote-User from a parameter,
not a header.