Keep reliability issue after participation was deleted

Based on !1888 (closed)

One issue with reliability is that we track it on the participation. That means this information is eventually deleted for subjects that dropped out.

I extended !1888 (closed) so that reliability issues are tracked on a separate model that is not deleted along with its participation.

Still to do: Currently Participation.would_reinvite is first created and then removed again, resulting in 2 additional migrations. I would like to rearrange the changes to avoid that.