Bug 19068: Add quantity field to the opac suggestions form

Bug 19068: Add quantity field to the opac suggestions form

This enhancement just exposes the quantity field to the OPAC suggestions
form to permit the end user to enter this if they wish.  The librarians
can of course override this quantity when they process the suggestion.

Test Plan (assuming using kohadevbox VM):
1) apply patch and turn on purchase suggestions
2) Go to http://localhost:8080/cgi-bin/koha/opac-suggestions.pl?op=add
and notice that there is now a quantity field available.
3) Make a purchasing suggestion using this form, including a quantity.
4) Log in as a member of library staff and go to Home › Acquisitions ›
Suggestions
5) Confirm that the previously entered suggestion is there, and that the
correct quantity appears in the "Acquisition information" section.
6) Edit the purchasing suggestion, change the quantity, save it and
check that the new quantity appears in the suggestion.

I've popped an ID attribute of "opac-suggestion-quantity" to the
surrounding <li> so that CSS can easily make this optional field
disappear (as suggested by cait on IRC).

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>