[Bug 836321] Parse about:support data if possible.

This is a lot easier to read than just dumping JSON on the page. I wasn't too excited about reviewing @fxa90id's version of this code (#1142), and figured it would be quicker to write it myself.

I added the "modified preferences" field. This was called a "non-critical field we don't have to show", but discussion on the contributor forum made me think it was a desired field.

r?