#780 return user to original page after login

Created by: das-g

connected to #780 (closed) (part of it, actually)

This redirects the user to the page from where they came to our /login page, after they've logged in using an external identity/authentication provider (currently OpenStreetMap).

With the help of the documentation this was surprisingly easy. Everything else was already in place. (The next= parameter when being redirected to our /login page is set by the django.contrib.auth machinery triggered by LoginRequiredMixin. The @login_required decorator on a function-style view would trigger a different mechanism to cause the same effect.)