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.
`resendInvite` includes the email address and role from the model as it currently exists in the store.  As long as the model has been `set` with the correct role prior to `resendInvite` being called it should be fine.