Remove django-mptt

[cmc333333]

We've been using django-mptt to describe the schema of our nested set implementation. Unfortunately, it makes relatively strict demands on which versions of Django it supports (and doesn't encode those in its own dependencies). This required us to stop supporting Django 1.8 before its LTS expired, which is against our policy.

It should be pretty simple to replace the bits of schema that we use and remove mptt altogether.