JME part: add support for expected variable names

see numbas/Numbas#280
Merge branch 'master' into organisation
custom tag model with 'official' field; tag admin actions

Tags can be marked as 'official', which means they will show up on the front page tag cloud (once I've made it)

This also adds two admin actions, "mark as official" and "merge tags". When tags are merged the tag which is most-used is kept. You can always go to that tag and change its name afterwards.