Make markdown notebooks editable

[mofojed]

We use markdown notebooks on the demo system. Would be nice to make them editable so they're 1) easier to create in the first place, and 2) User's can play with them.

Perhaps we could make the code cells themselves editable as well as editing the full document.

[alexpeters1208]

I think the demo system should have editable code cells - I am indifferent to whether the markdown text is editable. If the demo is supposed to be a sandbox where people can play with our stuff, editable code cells are a big part of that. Otherwise, the IDE half of the demo system is a glorified pdf.