refactor: define user groups in an extra file

src/things/message.ts

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2019 StApps
+ * Copyright (C) 2019-2021 StApps
  * This program is free software: you can redistribute it and/or modify it
  * under the terms of the GNU General Public License as published by the Free
  * Software Foundation, version 3.
@@ -22,6 +22,7 @@ import {
 } from './abstract/creative-work';
 import {SCThingMeta, SCThingType} from './abstract/thing';
 import {SCThingThatCanBeOfferedTranslatableProperties} from './abstract/thing-that-can-be-offered';
+import {SCUserGroup} from './abstract/user-groups';
 import {SCOrganizationWithoutReferences} from './organization';
 
 /**
@@ -39,7 +40,7 @@ export interface SCMessageWithoutReferences
    *
    * @filterable
    */
-  audiences: SCMessageAudience[];
+  audiences: SCUserGroup[];
 
   /**
    * When the message was created
@@ -90,14 +91,6 @@ export interface SCMessage
   type: SCThingType.Message;
 }
 
-/**
- * Audiences for messages
- */
-export type SCMessageAudience =
-  'students'
-  | 'employees'
-  | 'guests';
-
 /**
  * Translatable properties of a message
  */