Redirected to custom path on successful signup (#12372)
Redirected to custom path on successful signup (#12372)

refs #12366 

This implements redirection based on the settings for successful member sign up!
- Removes support for redirecting to `req.path` afterwards, this was never used and
  we now have a more configurable implementation.
- Retains redirection to the homepage for unsuccessful sign up (invalid/expired token)
bitjson/Ghost,e10/Ghost,ErisDS/Ghost,e10/Ghost,ErisDS/Ghost,bitjson/Ghost,TryGhost/Ghost,janvt/Ghost,TryGhost/Ghost,TryGhost/Ghost,ErisDS/Ghost,janvt/Ghost