Fix registration template permissions

This depends on #1729.

Bug:
* If you didn't have permission to register (say, logged out), Django was forwarding to the wrong URL (settings.LOGIN_URL), as `securesync` is now using the `login_required` header for auth.

Expected:
* Get forwarded to our app-specific login URL (`/securesync/login`)

Changes:
* Given our older version of Django, I had to hard-code the login URL.  

Newer versions of Django would allow a named URL (for use with `reverse`), which would be nice.  Oh well!

Testing:
* Access `/securesync/register/` and made sure it forwarded to `/securesync/login/` rather than `/accounts/login/`, the Django default.