Add side-by-side Markdown previewer in editor

It would be nice to have a Markdown previewer of the content as it's written, displayed alongside the editor.

For the microblog post type, the preview could be styled to look like a tweet/social post, while notes and blog posts get a normal article-style preview.

The issue does not name files, tests, or entry points. Start by locating the editor implementation and the rendering paths for microblog, note, and blog post types; done means a side-by-side Markdown preview is available with the specified styling for each type.

Written by the indexing model from the issue text.