Auth redirects

[eritbh]

Implement smart redirects after logging in, e.g. when you point someone to /auth/reddit, specify a path they get sent to afterwards, e.g. /auth/reddit?to=/somewhere to log in and then redirect to /somewhere.

I don't think we actually need this for the site to work properly; only work on this if you're bored and don't think you could tackle any of the other open issues.