feat(rxAccountUsers): Use nav isActive

**NOTE**: This PR is against the `rx-account-users` branch, not against `master`

Rather than checking for `/cloud/` in the URL, take advantage
of the `key: 'cloud'` we use in the nav, and check if that
nav item is currently active
Note that `isCloudProduct` is no longer a method, it's a variable bound to the scope.