Prevent transition to signup on invalid invitation

Refs #3876
* Prevent signup page from flashing when an invalid invitation
  token is used.
* Clear sensitive information from signup controller.
* Make isInvitation API behavior consistent with other auth
  related APIs.

Example:
![signup](https://cloud.githubusercontent.com/assets/214142/4330916/e1057862-3fb9-11e4-8a9a-0ae91c888b1a.gif)