LP#1513554 - Prevent deletion of reserved rows.

LP#1513554 - Prevent deletion of reserved rows.

It was previously possible to delete reserved rows in
the acq.cancel_reasons table via the UI.  We take a belt
and suspenders approach here:

1) Grey out the checkboxes beside cancel reasons with an
   ID lower than 2000.
2) Create a trigger on the acq.cancel_reason table that
   prevents deletion of rows with an ID lower than 2000.

The trigger executes a new generally available function
for use in similar situations in the future.

Signed-off-by: Chris Sharp <csharp@georgialibraries.org>