Things break when a file name contains emoji

[dunstan]

I put an emoji in the filename and the preview of the emoji page rendered but the command line wasn't happy. I was also able to deploy it, but when I tried to view the emoji page on staging I got a "not found" error.

What do we think should happen here? Is this a bug?

@Fil also notes that he can put a < character in a file name.

[dunstan]

[Fil]

When a file is named a<b.md preview works, but hot reloading is broken.

[Fil]

related #778