syntax highlighting in github commit message boxes

GitHub does some syntax highlighting when rendering commit messages, we should match its behavior whenever a user is typing into a commit textarea.

Hardest part is just that GitHub uses a monospace font, different than all of the other textareas we are enhancing. If we finish

- #99

then implementing this would be pretty easy.

Some important fact-finding we need to do first: what aspects of markdown does GitHub render?

Start by documenting which Markdown constructs GitHub highlights in commit message textareas, then compare that behavior with the existing textarea enhancement and the prerequisite in #99. Done means commit-message input uses a monospace presentation and matches the agreed GitHub highlighting behavior.

Written by the indexing model from the issue text.