Bug 27297: Add required and blank options to av-build-dropbox.inc

Bug 27297: Add required and blank options to av-build-dropbox.inc

This patch adds two options to av-build-drobx
required - which will add the required attribute to the select
blank - which will add a blank option to avoid preselecting others

To test:
1 - In OpacSuggestionMandatoryFeilds select itemtype
2 - Go to OPAC
3 - Add a suggestion
4 - Don't fill out any fields
5 - Submit
6 - Suggestion is added
7 - Delete it
8 - Apply patch
9 - Add a suggestion
10 - Note itemtyp edefaults to 'None'
11 - You cannot submit until you select a value

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>