Fix login and edit links.

I set up a private installation of JSBin earlier today and noticed two issues:

1. The "login / register" link points at `/login`. This doesn't work when JSBin is running with a prefix.
2. The "Edit in JSBin" button on the output page didn't work properly. When running JSBin under `/jsbin` it would actually point at `/jsbin/jsbin/...`. 

These changes should fix both issues.