Replace old method for deleting store posts

Part of #9 (closed)

What does this MR do?

Replaces an old XHR method for deleting posts on store walls by a REST endpoint. The functionality shouldn't change at all.

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

Very sure

Links to related issues

• https://sentry.io/share/issue/30f9f6073da144499cb192f52d349cf1/

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. Go to a store
4. Post something on the store's wall
5. Delete the post

Screenshots (if applicable)

Any relevant screenshots if this is a design / frontend change

Checklist

• added a test, or explain why one is not needed/possible... still covered by existing tests.
• 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