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/channels/app/user.go

@@ -282,8 +282,9 @@ func (a *App) createUserOrGuest(c request.CTX, user *model.User, guest bool) (*m
 
 	recommendedNextStepsPref := model.Preference{UserId: ruser.Id, Category: model.PreferenceRecommendedNextSteps, Name: "hide", Value: "false"}
 	tutorialStepPref := model.Preference{UserId: ruser.Id, Category: model.PreferenceCategoryTutorialSteps, Name: ruser.Id, Value: "0"}
+	gmASdmPref := model.Preference{UserId: ruser.Id, Category: model.PreferenceCategorySystemNotice, Name: "GMasDM", Value: "true"}
 
-	preferences := model.Preferences{recommendedNextStepsPref, tutorialStepPref}
+	preferences := model.Preferences{recommendedNextStepsPref, tutorialStepPref, gmASdmPref}
 	if err := a.Srv().Store().Preference().Save(preferences); err != nil {
 		c.Logger().Warn("Encountered error saving user preferences", mlog.Err(err))
 	}