866 add database constraint to fs_faq

Closes #862

What does this MR do?

add database constraints to table fs_faq

How confident are you it won't break things if deployed?

very confident

Links to related issues

n.a.

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. execute ./scripts/dropdb if needed
3. execute ./scripts/initdb
4. check in phpMyAdmin that constraints to fs_faq_category and fs_foodsaver are created in table fs_faq

Screenshots (if applicable)

n.a.

Checklist

• no functional change; only database maintenance; therefore no dedicated test required
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings