Add a WYSIWYG-esque editor for instructions

There are good pseudo-WYSIWYG editors (e.g. **bold text** is displayed bold etc.) for Markdown out there. A couple of good looking options are SimpleMDE and TOAST UI Editor, although both use CodeMirror rather than ACE (we use the latter for the main editors so it would be nice to share the dependency, although this is not a deal breaker).

Note that one thing we don’t want to do is load a Markdown editor library synchronously as part of the initial pageload—I think we will want to load it async and only if/when editing mode is activated. This might be a little tricky, especially before React releases support for async components, but I’m happy to share some ideas.

Review the existing ACE-based main editors and the instruction editing flow first. Compare SimpleMDE and TOAST UI Editor, then define the editor behavior and loading approach. Done means instructions have a WYSIWYG-style Markdown editing experience without loading the Markdown editor synchronously during the initial page load.