Improved Markdown editor

[btrask]

Our Markdown editor is pretty bad.

From what I've seen StackEdit might be the best option, but I don't know how it handles Common Markdown or how easy it would be to integrate our changes.

If we want to fix our current editor, here are some of the most obvious issues:

• Inserting tabs is really slow and janky on large files
• Scroll sync between editor and preview is bad (in theory this should be easy because cmark gives us "source positions" in its output, but in practice it's still hard because we can't get the source position from the textarea...)