Fixed invite management tests
Fixed invite management tests

no issue

- adjusts mirage config so that we're correctly serializing foreign keys
- our API outputs (and our app code expects) foreign keys for non-embedded resources to be in the format `{relationship_name}_id` but mirage's default REST serializer does not include the `_id` suffix
- tests started failing because https://github.com/TryGhost/Ghost-Admin/commit/c46d04f6120302e1e73edb473f04b4108328672d introduced a direct access of the invite->role relationship which Ember Data was not correctly creating when mirage was outputting `role` rather than `role_id`

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost