LP#1835982: add cell value print handlers to holdings grid

LP#1835982: add cell value print handlers to holdings grid

This patch adds cell value print handlers for the barcode
and holdable columns of the Angular staff catalog holdings
view.

To test
-------
[1] After applying the patches for this bug, perform a "Download
    Full CSV" or "Print Full Grid" action on the Holdings View
    grid in the Angular staff catalog.
[2] Verify that the Location/Barcode and Holdable? columns in the
    output contain appropriate values.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>