Add API for store categories

First part of #1263 (closed). I would like to add the frontend in another MR.

What does this MR do?

Adds API endpoint for listing, adding, editing, and removing store categories. Permissions are limited to orga for now. The API can be later used in the frontend.

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

Quite sure. I added unit tests.

How to test

Can not really be tested manually, but it should be ok if the pipeline passes.

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• set the "API change" label if changes in the API are not backward compatible
• added to the next milestone (see https://gitlab.com/foodsharing-dev/foodsharing/-/milestones, unless it has a "for:Dev" label)
• added an entry to CHANGELOG.md
• added a short text in the release notes to /release-notes/YYYY-MM.md
• Once your MR has been merged, you are responsible to create a testing issue in the Beta Testing forum: https://foodsharing.de/region?bid=734&sub=forum. Please change the MRs label to "state:Beta testing".
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...) how this change can be tested.
  • Be aware, that also non technical people should understand.