Fix null sort order error and date drift on Animal Group Management Interface

[jallentxbiomed]

Fixes two bugs on the Animal Groups data-entry form (animalGroupCategories):

1. "Update failed: Unable to convert value 'null' to Integer" when saving a row with a blank Sort Order

• AnimalGroupsController.UpdateGroupsAction was passing the JSON null sentinel straight to the row map. Guarded sort_order with o.isNull("sortOrder") so a blank field is sent as a real SQL null (the column is nullable).

2. Start/End date drifting back one day on every edit (timezone round-trip)
   This needed a coordinated server + client change; neither half fixes it alone:

• Server (AnimalGroup.toJSON): format date and endDate with DateUtil.formatIsoDate(...) so the payload carries a bare Y-m-d string instead of a zoned datetime.
• Client (AnimalGroupsStore.js): added dateReadFormat: 'Y-m-d' to the date and endDate fields so ExtJS parses that string as a local date rather than UTC midnight (which shifted it to the prior day in Central time).