Simple helper to read out emailDomain

Makes it tidier to check the domain:

``` scala
  override def validateUser(authedUser: AuthenticatedUser): Boolean = {
    (authedUser.user.emailDomain == "guardian.co.uk") && authedUser.multiFactor
  }
```

vs the current:

``` scala
  override def validateUser(authedUser: AuthenticatedUser): Boolean = {
    (authedUser.user.email endsWith ("@guardian.co.uk")) && authedUser.multiFactor
  }
```