Add /authorize link support

I needed a way to allow users to reauthenticate if they disconnected the app from Foursquare’s side or want to switch accounts.

Per https://developer.foursquare.com/overview/auth, “Using `/authorize` will ask the user to re-authenticate their identity and reauthorize your app while giving the user the option to login under a different account.”

So I added `AuthorizeLink()`, distinct from `AuthenticationLink()`.