Issue #4: Switch over to Poetry for dependency management

This PR implements part of issue #4 by moving dependencies over to poetry and committing migrations files to source control. FYI also turned up a piece of code that needed to be updated to support the most recent 3.x version of Django Rest Framework.

For the time being I ended up just merging all the requirements.txt files into one to simplify local dev. For final production deployment we may need to re-disambiguate the two packages, but sounds like that's a bridge that can be crossed later.

@jamie I tested this on buster inside of a Docker container and it seemed to work smoothly, but curious to hear how it goes for you!