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.
Nevermind, I just looked at what the resendInite function actually does, namely:

```
var fullUserData = this.toJSON(),
            userData = {
            email: fullUserData.email,
            roles: fullUserData.roles
        };
```

We're good here :+1: