Make GM behave as DM (#24289)

* Make GM behave as DM

* Fix lint

* Add desktop notification special behavior

* Change notification preferences menu

* Make changes to the GM channel intro

* Fix tests

* Fix i18n and style lint

* Add system notice and update style

* Fix style and fix tests

* Fix tests

* Handle push notifications as desktop notifications

* Fix tests

* Add test and default GMs to none when user level config is none

* Fix test

* Update only for mentions text

* Add tests

* Fix lint

* Fix lint

server/public/model/preference.go

@@ -31,6 +31,8 @@ const (
 	PreferenceNameUseMilitaryTime         = "use_military_time"
 	PreferenceRecommendedNextSteps        = "recommended_next_steps"
 
+	PreferenceCategorySystemNotice = "system_notice"
+
 	PreferenceCategoryTheme = "theme"
 	// the name for theme props is the team id