Fixes #3467 - Redirect old /signin, /signup, /forgot to the homepage

I've modified the REDIRECTS and redirect() function so that we can
also specify whether the redirect is a 301 or a 302. Prior to this
we were only able to do the default 302 redirect.
Merge pull request #3594 from mozilla/issue-3467

Fixes #3467 - Redirect old /signin, /signup, /forgot to the homepage