Deprecate the `SectionTreeAdminMixin`. It's better `mptt` practice to use TreeForeignKey/TreeOneToOneField/TreeManyToManyField explicitly in the model definition.

- I don't believe we've ever used this in Armstrong itself so I don't think there is anything else to change.
- drop EMPTY_LABEL. It doesn't appear necessary.
- Fix: using EMPTY_LABEL in `formfield_for_manytomany()` was actually breaking
development env - fix static files use in newer Django's and provide required `STATIC_ROOT` setting. Drop Sites. Add some "no covers" for Coverage.
Coverage - increased to 100%. Fix duplicate test case name.