Do not rate contests that have not yet been rated when DQ'ing users

The current behaviour is that when a contestant is DQ'd from a contest, all contests that end after said contest, but before the current point in time, are re-rated.

However, this does not account for the fact that some of these contests have not yet been rated themselves.
This is a bug: and the DQ code should only re-rate contests that have previously been rated, not rate new contests.