Infrastructure update

[rebeccacremona]

This PR:

• Changes the file structure of the repo a bit, to make it more like recent LIL projects. Notably, one of the directories named perma-payments is now named web. I did NOT change the Django app from perma_payments to our now-usual main, because that would have required extensive changes across most files, and would affect the db and migrations; I don't think it's worth it.

• Switches from pipenv to pip-tools, and, to facilitate that, switches the base Docker image from plain debian:buster to python:3.7-buster

• Upgrades to Django 2.2. (I stopped there, rather than going all the way to Django 3.0 or 3.1, because 2.2 is the current LTS version, and Jack recommends sticking with those.)

• Upgrades the rest of python packages

The changes to the file structure and the move to pip-tools will certainly require changes to our deployments and deployment scripts... I think it's worth it, so that, after I similarly alter Perma, all our projects use pip-tools.

[coveralls]

Coverage increased (+0.1%) to 95.276% when pulling 233cbf7 on rebeccacremona:updates into f98c6bd on harvard-lil:develop.