fix(grapher): allow saving/recovering stackMode config property
fix(grapher): allow saving/recovering stackMode config property

This reverts a part of https://github.com/owid/owid-grapher/commit/1581f9d2bdd39bb65d74d4a6973f4bddc6f6c32f.

It's a quick fix for a bug we have where `stackMode` is not being saved/recovered at all from config.

The logic as it was written also wasn't quite right: charts can be stacked by either metric or entity, so that needs to be taken into account. But additionally, that information is stored in the `seriesStrategy`, which doesn't have a consistent meaning from chart to chart.

TL;DR: This likely needs some agreement on what `seriesStrategy` means between charts, and some refactoring.

OurWorldInData/owid-grapher,owid/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,OurWorldInData/owid-grapher,owid/owid-grapher,owid/owid-grapher,owid/owid-grapher,owid/owid-grapher