Add reporting command for visitors with invoices or invoices without visitors

config-example.json

docs/source/development.rst

stripe_checkout/config.py

stripe_checkout/settings/base.py

stripe_checkout/stripe_checkout/compare_visit_stripe.py

stripe_checkout/stripe_checkout/generate_report.py

...out/stripe_checkout/management/commands/generatereport.py

...kout/stripe_checkout/management/commands/processevents.py

...stripe_checkout/management/commands/sendpossibleerrors.py

stripe_checkout/stripe_checkout/visit.py

test/conftest.py