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/api4/preference_test.go

@@ -50,8 +50,8 @@ func TestGetPreferences(t *testing.T) {
 	prefs, _, err := client.GetPreferences(context.Background(), user1.Id)
 	require.NoError(t, err)
 
-	// 5 because we have 2 initial preferences tutorial_step and recommended_next_steps added when creating a new user
-	require.Equal(t, len(prefs), 5, "received the wrong number of preferences")
+	// 6 because we have 3 initial preferences tutorial_step, recommended_next_steps and system_notification are added when creating a new user
+	require.Equal(t, len(prefs), 6, "received the wrong number of preferences")
 
 	for _, preference := range prefs {
 		require.Equal(t, preference.UserId, th.BasicUser.Id, "user id does not match")