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

* feat: mattermost integration

* post review adjustments

packages/backend/src/apps/mattermost/common/get-base-url.ts

@@ -0,0 +1,7 @@
+import { IGlobalVariable } from '@automatisch/types';
+
+const getBaseUrl = ($: IGlobalVariable): string => {
+  return $.auth.data.instanceUrl as string;
+};
+
+export default getBaseUrl;