feat(mattermost): add auth and send message to channel action

* feat: mattermost integration

* post review adjustments

packages/backend/src/apps/mattermost/common/get-current-user.ts

@@ -0,0 +1,9 @@
+import { IGlobalVariable, IJSONObject } from '@automatisch/types';
+
+const getCurrentUser = async ($: IGlobalVariable): Promise<IJSONObject> => {
+  const response = await $.http.get('/api/v4/users/me');
+  const currentUser = response.data;
+  return currentUser;
+};
+
+export default getCurrentUser;