Use poetry for python dependency management

[dchiller]

This PR changes the way Cantus Ultimus handles dependencies. In the process, it closes #734.

Change introduced:

• python dependencies are managed with the poetry package. These are now configured in the pyproject.toml file and locked with the poetry.lock file. A dev dependency group is introduced that includes python dependencies useful for development when installed outside the app container but unnecessary for deployment; these can now be standardized across development environments. The README is updated to account for these changes.
• The build process of the app container has been adjusted to account for the use of poetry.
• Previously, python dependencies were installed as part of the nginx container build in order to collect Django-managed static files to a location in that container. This PR removes the need for python in the nginx container build process by collecting static files in the app container, and sharing them via a new volume with the nginx container. As I result, I'm going to say this closes Optimize nginx container build #552. No other optimizations currently seem available.

NOTE: The django-extensions (and by extension, its dependency Werkzeug) are in fact development dependencies, but need to be installed in the app container. I've opened #839 to address this at a later date.

[jacobdgm]

I don't know enough about Poetry for my approval to carry any weight, but I've looked through it and there don't seem to be any egregious errors.