LP#1800178 Holdings View should also sort by part

LP#1800178 Holdings View should also sort by part

The Holdings View tab of the web client doesn't correctly sort by
monograph parts. The JS is already grabbing the part label for display,
so this commit moves that chunk before the sorting code, and also grabs
the sort key. Note that the part labels and sort keys are joined per
item, since the database supports multiple parts per item (though the UI
hasn't yet supported that). And we now join the label on comma-space,
instead of the default (just a comma).

Signed-off-by: Remington Steed <rjs7@calvin.edu>