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 cc0aa73355 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`

mirage/serializers/application.js

@@ -19,6 +19,10 @@ export default RestSerializer.extend({
         return underscore(relationship);
     },
 
+    keyForForeignKey(relationshipName) {
+        return `${underscore(relationshipName)}_id`;
+    },
+
     serialize(object, request) {
         // Ember expects pluralized responses for the post, user, and invite models,
         // and this shortcut will ensure that those models are pluralized