webstaff transit list: switch to egGridFlatDataProvider

webstaff transit list: switch to egGridFlatDataProvider

This gets us sorting "for free", or at least more easily than
with a custom-written data provider.  The following fields
are marked sortable for now:

- barcode
- title
- source library
- destination library
- send date/time

Because egGridFlatDataProvider gives us flattened hashes, not
fieldmapper objects, various tweaks were required to compensate.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>