[MSE] PSA: The MSE spec is now on GitHub

I've just finished migrating the MSE spec to GitHub.
Please use https://w3c.github.io/media-source/ when referencing the
Editor's Draft.

The old MSE spec URLs on the dvcs server now redirect to GitHub. I've also
removed the -respec.html sources from that repo so that there is no
confusion about which files are use to publish the spec. I also updated all
the links in the Revision History section to point to the eqivalent GitHub
revisions instead of the the dvcs.w3c.org server.

I would also like to thank Mike Smith for getting us set up on GitHub and
answering all my questions.

Aaron