add flag to toggle check for expected variable names

The check for expected variable names is only done when the "Warn if student uses an unexpected variable name" box is ticked.
When it is ticked, an empty list of expected variables means that any variable will trigger the warning. (this is different from before, when an empty list would mean the check wasn't done at all)

see #280
update jme-display.js
fix htmlparser to allow hyphens in attribute names
don't let TinyMCE mess up HTML
Merge branch 'master' into organisation
fix tag migration
tag cloud on the question index

shows official tags. Size is assigned based on how many questions use the tag.