Recalc blackout height when injection is shifted down & don't create a gap when there isn't one

The change on line `509` deals with the issue found on this post: https://github.com/GigaOM/gigaom/issues/5848#issuecomment-61326891

The rest relate to issue in the description of this ticket: https://github.com/GigaOM/gigaom/issues/5848