Support for disabling bumping neighboring blocks

Laying the ground for the fix for https://www.pivotaltracker.com/story/show/70791820.

If you drag and drop a group of blocks near another block without attaching it, some of the blocks can end up getting bumped so that non attached blocks don't overlap at all.  This never seems like the desirable behavior for Jigsaw.  Instead, we will just always disable this bumping.