Set up linter on the new-django-app template

[jeancochrane]

Documentation request

Similar to #75, we have some ad-hoc style conventions for Python but our typical tool for automatically enforcing them (flake8) is not built into our templates and is absent from many of our apps. Let's update our new-django-app template to install a linter, including:

• Updated requirements.txt file installing the linter
• Default configuration file
• GitHub Action workflow to run the linter on commits to pull requests
• Boilerplate documentation in the README explaining how to run the linter

[jeancochrane]

Another consideration: Can we configure flake8 to run using the container image defined in a repo's Dockerfile, as opposed to the system Python?

[hancush]

Our fork of pytest-flake8 no longer works as of pytest 6.0. If we want to use the plugin, we'll either need to patch it or pin pytest<6.0. @fgregg also suggests eschewing the plugin and running flake8 as a separate test step. TBH, I like that, too, since the plugin has proven tricky!