Redirect user back to the original page when they sign in

Created by: sqs

From @JoshuaKGoldberg:

User login flow: I ran a basic search anonymously, clicked Log In, logged in with Github, and was redirected to the Sourcegraph homepage. The query was lost. The Github integration did have to ask for new permissions on my account. Maybe that's relevant?

I've also experienced this myself. We should redirect the user back to the page they were on (with security guidelines in place to forbid open redirects, etc.) when they sign in.