Fix redirect to login page when using single open id provider authentication

This pull request ensures that when accessing a link directly (e.g. going to the editor from a bookmarked link) under single open id provider authentication, Zanata will not go to the usual login page, but instead will go directly to the provider.

Things to test:
- [x] Login button works for internal and open id configurations (both single and multiple provider)
- [x] Accessing an unauthenticated protected url on internal auth should redirect to the login page.
- [x] Accessing an unauthenticated protected url on single provider open id auth should redirect to the provider's page.
This name makes this look like an event handler, which suggests that it would perform some action, but all it does is return a String. Its purpose is to provide an identifier for which page to redirect to, so why not `getLoginPageId`?