Avoid double-counting the /sign_in page when users return from authenticating with their primary.

I am slightly uncomfortable with this and am opening to get feedback from @6a68, @kparlante and @seanmonstar.

* Instead of using the #AUTH_RETURN hash, use ?AUTH_RETURN, which is sent to the backend and can be used to avoid double count.
* Update tests.
* Get rid of the testmob.org include, it has been reaped.

fixes #3970