Update foodsharepoint and store bells instead of adding new ones

Closes #719 (closed)

What does this MR do?

This merge request

• enables bell notification updates to change the foodsavers seeing the bell
• makes creating store feed posts part of the REST API
• makes store posts and food share point posts update old bells instead of creating new ones to reduce the amount of new bell notifications to one per store and per food share point
• doesn't mark the store post bell notification as unread for the posts author

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

Uhm... quite. If the xhr method for creating new store posts, which I removed, has not been used by any app besides the foodsharing.de frontend, it should not break anything.

Links to related issues

#271 (closed)

Checklist

• added a test, or explain why one is not needed/possible...
• 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