Make sensitivity controls adjust according to user-bell/wheatley-bell ratio

[kneasle]

If few human ringers are ringing, the sensitivity controls should automatically be adjusted towards having Wheatley be less sensitive to human ringers.

[centreboard]

Does #66 help with this or needs more investigation?

[kneasle]

#66 does help with this, but it isn't a full solution. I think that in order to make a start on this, we need an easier way to debug Wheatley's internal regression state, since e.g. slight but accumulating slowdowns will be extremely difficult to spot.

But yes, this does need more investigation. But I think having merged #66 and adding thresholds for when bells are classified will make a pretty decent solution.