Set numbasVersion in editor.js; use it in exams!

I forgot to set Editor.numbasVersion for the exam editor. Anyway, it doesn't make sense to set it separately for questions and exams, so now it's set in editor.js
Add "replace with gapfill" button for top-level parts

Parts which are not already gaps but could be (i.e., not information only) now have a "replace with a gap-fill" button next to the "copy part" button.
This replaces the part with a gap-fill whose first gap is the old part. The prompt of the gap-fill is copied over from the old part, with a [[0]] placeholder at the bottom to include the part-which-is-now-a-gap.

Fixes #215
add interface for "pick N questions" option

see numbas/numbas#351