this just breaks if we need to use "aliased" models e.g. when sorting
and/or filtering by Product "regular price" column and similar. so
now sorting more like we always used to, except for multi-column.
nb. this still assumes callers use `Grid.make_sorter()` when declaring
the sorters. if caller must specify more custom/explicit sort logic
then it likely will not work and we'll have to add a workaround to
allow avoiding the common logic..but that's another day
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