Bug 10141: improve editing of MARC21 bib 008/18-21 and 008/24-27

Bug 10141: improve editing of MARC21 bib 008/18-21 and 008/24-27

This patch changes the MARC21 008 editing form to create dropdowns
for each of the 008/18-21 (Illustrations) and 008/24-27 (Nature of contents)
positions.

To test:
1) Edit or add a new bibliographic record
2) Click on field 008 value builder, positions 18-21 and 24-27 only
show an input text box
3) Apply the patch
4) Reload value_builder page, now each position (18-21,24-27) has
its own dropdown.  Note that it may be necessary to clear
the browser cache for the change to be visible.

Similar changes could be made for the 006 value builder.

Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>