LP#1361801: Add required fields to serial issuance form

This commit prevents issuances from being created without a date_published
or holding_code. This has two benefits:

  1. Prevents uncaught exceptions related to submitting null values for
     either of those fields.
  2. Requiring date_published prevents an ugly error when trying to
     predict new issuances from an issuance without a date_published.

This commit does NOT provide any feedback to the user about which fields
are required. That would require further investigation into the Dojo/Dijit
objects being used for the edit form fields.

Signed-off-by: Remington Steed <rjs7@calvin.edu>
Signed-off-by: Jennifer Pringle <jpringle@sitka.bclibraries.ca>
Signed-off-by: Ben Shum <bshum@biblio.org>