RUSTSEC-2021-0063

[comrak](https://github.com/kivikakk/comrak) operates by default in a "safe" mode of operation where unsafe content, such as arbitrary raw HTML or URLs with non-standard schemes, are not permitted in the output. This is per the reference GFM implementation, [cmark-gfm](https://github.com/github/cmark).

Ampersands were not being correctly escaped in link targets, making it possible to fashion unsafe URLs using schemes like `data:` or `javascript:` by entering them as HTML entities, e.g. `&#x64&#x61&#x74&#x61&#x3a`. The intended behaviour, demonstrated upstream, is that these should be escaped and therefore harmless, but this behaviour was broken in comrak.