Resolve "Bell for updates on event wall"

Closes #228 (closed)

What does this MR do?

Adds a bell for users who intend to (maybe) be part of an event when there is a new wall post.

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

!3337 (merged) must be merged into prod before this merge, otherwise the bell translations for bells created in Beta would cause erros when read in prod.

How to test

1. Create an event with at least 2 (maybe) attendees
2. Write to the wall
3. Check bells with other user

Screenshots (if applicable)

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.