Bug 18011: Enrollment period date on patron category can be set in the past without...

Bug 18011: Enrollment period date on patron category can be set in the past without any error/warning messages

This patch modifies the patron category edit form so that the enrollment
period datepicker widget cannot be used to select a date in the past.

The <input> element had a "datepicker" class, which triggers a default
datepicker configuration. This was overriding the configuration in the
page-specific JavaScript file.

To test, apply the patch and go to Administration -> Patron categories.

Click the "Enrollment period -> Until date" field and verify that dates
before today are disabled.

Signed-off-by: Pierre-Marc Thibault <pierre-marc.thibault@inLibro.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>