Make date in event a mandatory field

Closes #436 (closed)

What does this MR do?

I set the date field to mandatory when creating an event. Since the issue description reads "Maybe don't check if the date is in the past since this is currently the only way to "delete" an event", I did not make any changes to that. Furthermore, I set the number of avatars to be shown next to an existing event from 52 to 54.

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

I only changed a few lines of code, hence I don't think the changes will cause troubles.

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible... --> I only made two minor changes and ran ./scripts/test
• 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))