Content - ContentTypes ContentTypeResource and ContentUnitsCollections

I thought these three would share more code than they ended up sharing so I combined them. The next one looks more complex though, so decided to make this PR now.
I agree Django's test runner would be good to move to because it would be less for us to maintain. I think that for now we're going to move these statements to run-test.py so that the settings.py module can be found in all tests. It will also deduplicate the os.environ call so that it's all in one place. My thinking is that we can focus on the port now and focus on improving the test runner as a separate issue. What do you think?