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/api/user.js

@@ -234,7 +234,6 @@ Cypress.Commands.add('apiCreateUser', ({
     prefix = 'user',
     createAt = 0,
     bypassTutorial = true,
-    hideActionsMenu = true,
     hideOnboarding = true,
     bypassWhatsNewModal = true,
     user = null,
@@ -265,10 +264,6 @@ Cypress.Commands.add('apiCreateUser', ({
             cy.apiDisableTutorials(createdUser.id);
         }
 
-        if (hideActionsMenu) {
-            cy.apiSaveActionsMenuPreference(createdUser.id, true);
-        }
-
         if (hideOnboarding) {
             cy.apiSaveOnboardingPreference(createdUser.id, 'hide', 'true');
             cy.apiSaveOnboardingPreference(createdUser.id, 'skip', 'true');