Make exercise pages nicer for working HW

[ngoodman]

It would be easier for students to work exercises in-page if we:

• Make code/text boxes persistent so that accidentally navigating away didn't lose work. (Would require use of localStorage in... webppl-editor? could be done in an extra js asset in the exercise layout?)
• Add md input boxes and put them as response fields where free responses are requested.
• Have some nice way to export the page. Is saving to pdf from browser sufficient?