Add initial functionality

[jmduke]

Remaining work before merging

• Write an actual README
• Replace BUTTONDOWN_APPS with a settings abstraction for declaring which apps have checkers
• Replace references to https://admin.buttondown.email with SITE_URL
• Replace json_table_widget.html's hardcoding of specific models with something more clever
• Remove dependence on requests, rich, related_admin, humanize, fieldsignals
• Replace reactions defaults with something more configurable
• Actual project/pip stuff
• Squash migrations

[nickmccullum]

if you ever do open-source this project fully and publish as a package, I needed to add this logic to handle the fact that we use UUID PKs not integer PKs, and the default UUID object is not JSON-serializable:

for failure in collected_failures:
    failure.data = json.dumps(failure.data, cls=DjangoJSONEncoder)

[jmduke]

Ironically i also use UUID PKs and run into this all the time and did not think to just serialize it at this level...