Bug 13007: Add a foreign key for suggestions.budgetid

Bug 13007: Add a foreign key for suggestions.budgetid

The DB field suggestions.budgetid should be linked to
aqbudgets.budget_id.

If the fund is removed, this column should be set to NULL.

Test plan:
1/ Using your SQL CLI (or equivalent), create or update 1+ suggestions and
set "0" in the budgetid field (or a nonexistent budget id).
2/ Execute the updatedabase script.
3/ Verify that your suggestion is unlinked to the nonexistent fund.
4/ Verify the constraint has correctly been added (show create table
suggestions).
5/ Check that this change does not affect the behavior on adding a
suggestion (linked to a fund or not).

Signed-off-by: Paola Rossi <paola.rossi@cineca.it>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>