Added in system check for 1.7+ which points out if I've been an idiot
and forgotten to add treebeard in.
Implement Django 1.7+ system check for the required setting
Without the setting, the import form raises ImproperlyConfigured
(and still will under < 1.7)
Change the import form to disable the "menu" dropdown if no choices were found.
Also swaps out the help text to explain why it's not possible to select anything.
Add a better verbose name to `uri`
`django-setuptest` never merged my 1.7 support
so we'll migrated away from it. No idea if this works in < 1.7, though :/