Upgrade to New Upstream

[FA-1009](https://freeagent.myjetbrains.com/youtrack/issue/FA-1009) is a bug to do with the Blink rendering engine causing a problem with newlines. It turns out that the FA codebase wasn't even using our forked version of this code (in master on this repo), but an older version. The our bug was also [referenced](https://github.com/xing/wysihtml5/issues/497) on the upstream project, which is apparently no longer maintained.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. 

Clear right?