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

e2e-tests/cypress/tests/support/index.js

@@ -261,7 +261,6 @@ function resetUserPreference(userId) {
     cy.apiSaveOnboardingTaskListPreference(userId, 'onboarding_task_list_open', 'false');
     cy.apiSaveOnboardingTaskListPreference(userId, 'onboarding_task_list_show', 'false');
     cy.apiSaveCloudTrialBannerPreference(userId, 'trial', 'max_days_banner');
-    cy.apiSaveActionsMenuPreference(userId);
     cy.apiSaveSkipStepsPreference(userId, 'true');
     cy.apiSaveStartTrialModal(userId, 'true');
     cy.apiSaveUnreadScrollPositionPreference(userId, 'start_from_left_off');