Use Markdown in review descriptions, issues and notes

[jacobrask]

When displaying review descriptions in the new frontend without <pre>, I've noticed that people already use basic formatting in descriptions, like lists. The same syntax for bulleted and ordered lists would be valid in Markdown.

If we use a decent Markdown library we could also add our own parsing for review ids and commits, and link to those.

[jensl]

Seems like a nice idea, but also probably something that should be optional, per system/user/repository/whatever.