Bug 13515: Add a foreign key constraint to messages.borrowernumber

Bug 13515: Add a foreign key constraint to messages.borrowernumber

This patch adds a missing foreign key constraint to
mesages.borrowernumber.
To create it sucessfully the entries from the messages table that are
not linked with an existing patron will be removed.

Test plan:
0/ Do not apply the patch
1/ Add messages to different patrons
2/ Delete one of the patron's record
3/ Have a look at the messages table and notice that the messages for
the deleted patron's record still appear
4/ Apply the patch and execute the update DB entry
5/ Have a look again at the messages table and notice that the messages
for the deleted patron's record have been removed
6/ Delete a patron that have messages
7/ Notice that now the messages are deleted when the patron's record is
deleted

Signed-off-by: Nazlı Çetin <nazli@devinim.com.tr>
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>