Logistration: Redirect to a valid URL when cancelling third party auth

[ECOM-637](https://openedx.atlassian.net/browse/ECOM-637): Third party auth cancel redirects to 404

The exception middleware in third party auth assumed that the "auth entry" corresponded to valid URLs without the prepended slash.  For the new login/registration page, we were using auth entry keys that were *names* of URLs, so needed to be reversed.

To address this, I have created an explicit mapping of auth entry points to dispatch URLs.  The exception middleware now checks this dictionary to determine the redirection URL.

Reviewers: @stephensanchez @dianakhuang 
It's just the way the old URLs are defined.  `/login` works, but `/login/` is a 404.