Switch Markdown rendering from Text::MultiMarkdown to CommonMark

[ilmari]

Custom rendering function to add id attribute to headings.

Fixes #2312

[haarg]

Generally looks good.

We'll need to wait to merge this until we can change our deployments to include libcmark. It also looks like it needs to get installed in CI.

[ilmari]

According to the docs I should be able to use sudo apt install libcmark-dev, but it fails with:

Run sudo apt install libcmark-dev
  sudo apt install libcmark-dev
  shell: sh -e {0}
/__w/_temp/3da801c7-faa1-4b90-90e5-c299fa68d890.sh: 1: sudo: not found
Error: Process completed with exit code 127.

[oalders]

The docker container has no sudo, so you can omit that from the command.

[haarg]

The content of this looks good. I've marked it as draft so that it doesn't get merged yet, until we can get the front end switched over to our new systems.