Revert remove slackUrl

Signed-off-by: Sebastian Malton <sebastian@malton.name>

packages/core/src/extensions/common-api/app.ts

@@ -53,5 +53,9 @@ export const App = {
 
     return di.inject(isLinuxInjectable);
   },
+  /**
+   * @deprecated This value is now `""` and is left here for backwards compatability.
+   */
+  slackUrl: "",
   issuesTrackerUrl,
 } as const;