coupons page: sort by expires_at by default

[smirolo]

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

Then browse to http://localhost:8000/billing/djaoapp/coupons/
The coupons should be sorted by expiration date.

The templates involved is saas/billing/coupons.html