allow multiple comments per participation

[jeriox]

closes #230 closes #1214

ToDo

• set author
• add previous comments as readonly to form
• proper design
• controls for visibility
• fix tests
• automatic comments for state change?

[coveralls]

coverage: 83.849% (-0.1%) from 83.97%
when pulling 9b1785e on participation-comment
into 3178e39 on main.

[jeriox]

@felixrindt I played around with the state change stuff a bit.
Finding the appropriate modellogging entries is easy: LogEntry.objects.filter(content_object_id=self.instance.pk, content_type=ContentType.objects.get_for_model(instance.__class__)))
But displaying them in a useful way would require merging them with the comments and sorting them afterwards. That is a lot of hassle and I'm not even sure anymore if it is that helpful. So I think I don't want to do that (at least not as part of this PR)