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.
This should actually be `deleteRecord`, since if the `save` failed there should be no record on the server to delete.

`deleteRecord` removes a record from the store.  `destroyRecord` is a `deleteRecord` followed by a `save` to persist to the backend.