enable gfm (github-flavored markdown) rendering of mdx blog posts

I noticed `"remark-gfm": "^3.0.1",` listed in `/package.json` but am unsure how to enable GFM rendering of `mdx` files in the template's `/posts` directory. For example, `- [ ] task lists` aren't rendered with checkboxes[^1].

[^1]: footnotes also don't work