Upgrade to New Upstream

There is a bug to do with the Blink rendering engine causing a problem with newlines. The bug was also [referenced](https://github.com/xing/wysihtml5/issues/497) on the upstream project, which is apparently no longer maintained.

However, a fix is available on this widely used fork: https://github.com/edicy/wysihtml5/. So, I've upgraded our fork with the Edicy version upstream.