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.
I think the first line should give a reasonable summary, before giving all the details. e.g.

```
/**
 * Indicates which page the user should be directed to when trying to log in.
 * ...
```