Move isolation et. al.

So, as requested, I've moved the whole stream isolation section from gUM to WebRTC.  In addition to that, I've added more text about isolation, in anticipation of an outcome to some of the discussions in D.C.   You might want to hold onto this until we've had that discussion.

I also fixed up [bug 25774](https://www.w3.org/Bugs/Public/show_bug.cgi?id=25774) while at it.

gUM was a mess whitespace-wise, so I cleaned that up first.