Changed middleware to try and get the Django admin URL dynamically

Pull request for #44. All tests pass, which is highly suspicious, because I've not added or modified any, and I feel like `MiddlewareTests.text_exclude` *should* fail, but I can't even get a `import pdb; pdb.set_trace()` breakpoint working when invoking `./run.sh test`, so it's actually kind of impossible for me to tell if my fix ... is a fix.