DoS via markdown API from unauthenticated user

`cmark-gfm` is GitHub's fork of `cmark`, a CommonMark parsing and rendering library and program in C. In versions prior to `0.29.0.gfm.6` a polynomial time complexity issue in `cmark-gfm`'s autolink extension may lead to unbounded resource exhaustion and subsequent denial of service. Users may verify the patch by running `python3 -c 'print("![l"* 100000 + "\n")' | ./cmark-gfm -e autolink`, which will resource exhaust on unpatched `cmark-gfm` but render correctly on patched `cmark-gfm`. This vulnerability has been patched in `0.29.0.gfm.6`. Users are advised to upgrade. Users unable to upgrade should disable the use of the autolink extension.