EventGateway fully implemented, EventModel removed

Treats #9 (closed)

What does this MR do?

Complete the implementation of the EventGateway, removed the EventModel.

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

because I'm kind of a newbie, I am not sure.

Links to related issues

Fixes #276 (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))