Cleaning up working branches

To make it easier for our next phase of work, I will soon be removing 
the 60 or so working branches we have been using, and replacing them 
with a smaller set of new branches just for the SC that made it into 
WCAG 2.1, primarily to support work on Understanding content. I will 
"archive" the old branches so it's possible to get at content in them if 
needed, but if you have content you want to retain I suggest you grab it 
now. You can create a private branch in your local repository, or copy 
content into a holding location of your choice. I don't expect this will 
be generally needed, but wanted to give warning before the branches 
disappear. No work should be going on in those branches, except a couple 
for which we're still sorting the final consensus.

I will follow up later with information about working with the new branches.

Michael