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.
I don't think `save` is needed?  What we need is for the roles property to be accurate on the model after the `store.find('user')` from up above, it doesn't need to be persisted to the database yet.