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()` makes a `POST` against the API with the current email address and roles from the model, so that takes care of making sure the database is up-to-date.

Unless I'm missing something the flow is as follows:

* Existing user is found in database.
* `invitedUser` model is updated with data from the `store.find()`
* `invitedUser.roles` may now be out of sync.
* Set `invitedUser.roles` back to what was chosen in the dropdown.
* Call `invitedUser.resendInvite()` to persist changed data (roles) and trigger a new invitation email.