active data extraction script should ignore releases that are only mapped to by completed scheduled changes

[bhearsum]

I noticed today that we have a whole bunch of recent releases in the latest production db dump - most of which are not currently mapped to. I think this is because we grab any releases that are mapped to by any scheduled rule change (https://github.com/mozilla/balrog/blob/752d1d548840f1753a8592af405846c6612f7f3c/scripts/manage-db.py#L137), instead of only incompleted scheduled rule changes.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1377460)