Draft: Resolve "Report button on an event"

UI

• this feature could be enabled by admins
• show a report button on each event if enabled (everyone could report)
• ask for a description of the issue for the reported event, this could be a dialog with a simple textarea I think.
• documentation
• send email to event author

what are the consequences of creating a report?

• send an email to every admin?

Closes #221 (closed)