Add option to redirect succesful logins based upon REQUEST_URI

At present, one can't set raven/login as Drupal's 403 page: one ends up in a riderect loop due to how drupal_goto() works. Thus, we add a config option to the module's admin page which can change the behaviour. With the option set, you can:
* set raven/login as the site 403 page
* click on a link for a node which requires the user to have authorization
* automatically be taken to the Raven login page
* on successful authentication, redirect the user back to the page they were trying to view