SearchKit - Sortable column fixes/improvements

[colemanw]

Overview

Fix sortable column headers on SearchKit admin tables.
Make click-sortable column headers configurable.

Before

Sortable column headers broken on SearchKit admin screens.
Not configurable which headers were click-sortable when building a table display.

After

Fixed, configurable.

[civibot]

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[eileenmcnaughton]

It seems to work & the optional click header works - I couldn't actually replicate the issue with it not working - but something like that was reported by a user

[colemanw]

I think it regressed in master, so you wouldn't have seen it in a release.