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:
- [ ] Login button works for internal and open id configurations (both single and multiple provider)
- [ ] Accessing an unauthenticated protected url on internal auth should redirect to the login page.
- [ ] Accessing an unauthenticated protected url on single provider open id auth should redirect to the provider's page.
This method does not actually go to a page, right? It just returns a string representing the page to go to. Could it be named something like `getLoginPageId` or `getLoginPagePath`?