Bug 18050: Add FK constraint on aqbudgets.budget_period_id

Bug 18050: Add FK constraint on aqbudgets.budget_period_id

This adds a FK constraint on aqbudgets.budget_period_id so that
a fund cannot be added with an invalid aqbudget.budget_period_id.

We should not have funds that belong to no budget. In case we have, the
update will be skipped and a note displayed.

Part1:
- Before applying the patch
- Make sure you have a budget with some funds linked to it
- You will have to change one of the funds to link to an invalid
  budget with SQL:
  UPDATE aqbudgets SET budget_period_id = 999 WHERE budget_id = max(budget_id);
- Apply patch
- Run updatedatabase - verify that you see the hint about 1 existing fund with invalid budget.
- Repair your fund with SQL
  UPDATE aqbudgets SET budget_period_id = ... WHERE budget_id = max(budget_id);
  (... needs to be your existing budget_period_id)
- Run updatedatabase again - verify it runs successfully now.
- If you try to change the budget_period_id to a non-existing now with SQL, you will
  get a database error. The new FK doesn't permit it.
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: Jonathan Druart <jonathan.druart@bugs.koha-community.org>