If feature flag is enabled, replace the old login/registration pages with the new combined login/registration page.
Add support for external auth on the combined login/registration page

Add support for redirect URLs in third party auth for combined login/registration page

Remove old login/registration acceptance tests

Add deprecation warnings to old login and register views

Move third party auth util to student_account

Add exception for microsites
Merge pull request #6077 from edx/will/enable-logistration

Login/Registration A/B test cleanup