4964: Use CommonMark, or at least allow adding Python-Markdown plugins


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What's the URL of the page this enhancement relates to, if any?

All Markdown text input fields

Describe the enhancement and the motivation for it.

Review Board currently uses Python-Markdown, which (religiously) implements the original Markdown spec from 2004, which has a number of issues.

Our users often run into such issues where Python-Markdown doesn't render things as expected. For example, indenting a sub-list to the level of the original list's contents renders it as part of the original list, rather than a sub-list as expected.

CommonMark is a newer standard that irons out a lot of these issues. It's used by GitHub, GitLab, Stack Overflow, Reddit etc.

This is a request for Review Board to switch to a CommonMark renderer.

Failing that, we'd at least like to be able to configure the Python-Markdown plugins that Review Board uses. This would let us avoid a lot of these issues by adding plugins such as "Truly Sane Lists" and "Breakless Lists".