aws-gate should have type annotations, so we can do more linting and discover design issues.

Since it's possible to create custom session documents it should be useful to have support for that.

On page http://localhost:8000/billing/djaoapp/coupons/
Currently the default sort order is -created_at which is useless.
The default sort should be by expires_at (coupons with no expiration dates listed first) and alphabetically second when the expiration date for two coupons are the same.

To test, run the following command to create coupons:
python manage.py load_test_transactions

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