Various improvements to preferences code (#27710)

* Add isPostFlagged selector

* Remove unused code for actions_menu preferences

* Improve types of Preference and getPreference

* MM-58111 Change makeGetCategory to take category name once

* Use shouldShowJoinLeaveMessages everywhere

* Change makeGetCategory to better memoize its result

* Fix test that needs EnableJoinLeaveMessageByDefault to be set

* Remove more references to action_menu preferences

webapp/platform/types/src/preferences.ts

@@ -5,7 +5,7 @@ export type PreferenceType = {
     category: string;
     name: string;
     user_id: string;
-    value?: string;
+    value: string;
 };
 
 export type PreferencesType = {