bug 3222: database changes for messaging preferences (DB rev 033)

bug 3222: database changes for messaging preferences (DB rev 033)

This patch modifies the database for the messaging
preferences enhancement <http://wiki.koha.org/doku.php?id=en:development:rfcs3.2:rfc32_defaultmessagingsettings>
as follows:

* adds a categorycode column to borrower_message_preferences

This allows a set of messaging preferences to be linked
to a patron category

* allow the borrowernumber column to be null

This allows a messaging preference set to be linked
*only* to a patron category.  If MySQL had check
constraints, I would add one so that exactly
one of borrowernumber and categorycode in a row in
borrower_message_preferences could be NULL.

* add a FK constraint from borrower_message_preferences.categorycode
to categories; if you delete a patron category, any default preferences
associated with it will be automatically deleted.

The messaging preference enhancement was sponsored by the Northeast
Kansas Library System (NEKLS).

Signed-off-by: Daniel Sweeney <daniel.sweeney@liblime.com>
Signed-off-by: Galen Charlton <galen.charlton@liblime.com>