add showCorrectAnswe option

see numbas/Numbas#322
examparser: don't strip tabs from line beginnings!
fix exam view when not logged in

get_profile() doesn't exist for the anonymous user
set dropZone for fileupload

fixes #201

dropZone specifies which elements should accept drag-and-drop events for the fileupload instance. This changes the ko handler so the dropZone is just the bound element.
requirements.pip gives minimum version numbers
resources tab gives the actual URL

the name of the uploaded file might not be the same as the original, for example if it's a duplicate filename. The resources tab now gives the full relative URL of the image, as used in the img src attribute.

and the URL is now `resources/question-resources/<filename>`, instead of `resources/images/<filename>`, because not all resources are images.
ImageDeleteView actually calls image.delete

oops. Maybe I assumed the base deleteview does that for me.
Merge branch 'master' into file-versions

Conflicts:
	editor/static/js/examparser.js
Merge branch 'file-versions' into templates

Conflicts:
	numbas/requirements.pip
templates gone; now there are variable groups

I decided that the templates thing wasn't really worthwhile. The template-style type-specific definitions are nice interface elements, so why not have them all the time? Once that's done, there isn't much point in the simplified mode - it just marks some variables as more editable than others.
You can do that by separating variables out into groups, so I've got rid of the templates and the variable list now separates variables into groups, which can be defined by the question author. It's a good way of encouraging authors to organise their variables, even if they're not writing a template.