required changes for adding constraint to store category in DB

Closes #862

What does this MR do?

adds database constraint between fs_betrieb_kategorie and fs_betrieb; applies required changes to store model and seed

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

very confident

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. Login as foodsaver
3. Add new store without category
4. Check in phpMyAdmin that fs_betrieb.betrieb_kategorie_id is NULL and not 0
5. Assign category to store
6. Check in phpMyAdmin that id is stored
7. Remove category from store
8. Check in phpMyAdmin that id is switched back to NULL

Checklist

• no change in overall functionality; only houskeeping change
• 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