MM-29703 Mark threads as read when channels are marked (#15994)

Co-authored-by: Jesús Espino <jespinog@gmail.com>

web/context_test.go

@@ -55,6 +55,8 @@ func TestMfaRequired(t *testing.T) {
 	th.App.SetSession(&model.Session{Id: "abc", UserId: "userid"})
 
 	th.App.UpdateConfig(func(cfg *model.Config) {
+		*cfg.AnnouncementSettings.UserNoticesEnabled = false
+		*cfg.AnnouncementSettings.AdminNoticesEnabled = false
 		*cfg.ServiceSettings.EnableMultifactorAuthentication = true
 		*cfg.ServiceSettings.EnforceMultifactorAuthentication = true
 	})