MM-14675: Eliminates implicit GroupChannel/GroupTeam associations. (#13199)

* MM-14675: Upserts GroupTeam when GroupChannel is linked.
MM-14675: Delete all associated GroupChannels when deleting a GroupTeam.
MM-14675: Using replica DB where possible.
MM-14675: Updates create method used in tests.

* MM-14675: Removes unnecessary DB retrieval of GroupSyncable record.

app/notification_test.go

@@ -275,7 +275,7 @@ func TestFilterOutOfChannelMentions(t *testing.T) {
 		constrainedChannel, appErr = th.App.UpdateChannel(constrainedChannel)
 		require.Nil(t, appErr)
 
-		_, appErr = th.App.CreateGroupSyncable(&model.GroupSyncable{
+		_, appErr = th.App.UpsertGroupSyncable(&model.GroupSyncable{
 			GroupId:    group.Id,
 			Type:       model.GroupSyncableTypeChannel,
 			SyncableId: constrainedChannel.Id,