Bug 13535: Add FK constraint on borrowernumber to alert table

Bug 13535: Add FK constraint on borrowernumber to alert table

When a patron is deleted, we should clean up the data connected
to the patron. This includes subscriptions to 'new issue arrived'
arrive messages from the serials module.

To test:

Part1:
- Before applying the patch
- Add a subscription and select a notice template for patron notification
- Go to the OPAC and subscribe to the email alert
- Log in as any other patron (not yourself) and subscribe with that patron too
- Verfiy on the subscription detail page, that the patrons are subscribed
- Note the borrowernumber of your patrons
- Delete the second patron
- Note borrowernumber and delete patron
- Run: SELECT * from alert; in a report
- Despite the patron being deleted, you will see entries for both
- Apply patch
- Run updatedatabase - verify no error message
- Run updatedatabase again - verify still no error message
- There should no longer be an entry with the deleted borrowernumber,
  but only the entry for your own patron account
Part 2:
- Start fresh with the web installer, verify there are no errors on
  creating the database tables

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>