Add stats to user list

[Thomas Kalka]

Is your feature request related to a problem? Please describe.

We use Hedgedoc in school. Sometimes one student is blocking all work for others. Statistics would help to identify malicious students.

Describe the solution you'd like

Count additions and deletions for each user and show them on the contributor listing.

Describe alternatives you've considered

In addition, malicious users could berate limited, line limited or limited to lines, which they created.