Check if user has already been invited

closes #3681
- A simple ember store check for the given email address is done before
the user is invited. Most of the code is actually unchanged, it’s
merely moved into a simple if/else.
If I'm not mistaken, calling `resendInvite` here could cause the user to be re-invited with a different role than was chosen in the dropdown:

* The API is queried for existing users and there's a hit on this particular user.

* `invitedUser.resendInvite()` is called and it builds up a `POST` request using data on the model.  Since the model has just been refreshed via the API it will have the `roles` from the previous invite which were stored in the database.

* The `POST` will be sent back with the same roles that came down from the API and may conflict with what was just selected in the role dropdown.