Add ?next= param for GitHub login
Add ?next= param for GitHub login

This fixes #1640 by adding `?next=/asd` query for GitHub login button.
It will redirect to pointed resource after successful log in.

swcarpentry/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,swcarpentry/amy,swcarpentry/amy,pbanaszkiewicz/amy