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.
`getLoginPageRedirect` sounds fine to me.