I should be able to revert a Thing to a previous state

Summary: From the History Summary (and possibly while inside View at Time), I should be able to select a Thing and say, "revert to this state".

Note that this story is to revert the entire Thing to that state, not just a single Property.

I believe this is lower-priority than undoing a single change, since Querki tends to think in terms of specific Property edits.