Make logistration generally available if feature flag is active

Makes logistration available at /login and /register as well as /accounts/login/ and /accounts/register/. In addition:

- Adds support for redirect URLs in third party auth for combined login/registration page
- Adds support for external auth on the combined login/registration page
- Removes old login and registration acceptance tests
- Adds deprecation warnings to old login and register views
- Moves third party auth util to student_account
- Adds exception for microsites (theming)

lms/static/js/student_account/views/AccessView.js

@@ -178,7 +178,7 @@ var edx = edx || {};
             this.element.scrollTop( $anchor );
 
             // Update url without reloading page
-            History.pushState( null, document.title, '/account/' + type + '/' + queryStr );
+            History.pushState( null, document.title, '/' + type + queryStr );
             analytics.page( 'login_and_registration', type );
         },