* Add a nullable billing_periods column in the coupons table
* Add nullable billing_periods column to the currently unused
coupon_codes table
* Drop the duration column from the coupon_codes table
* Replace duration config type so that the default promotional coupon
can be configured to never expire
Zero downtime migration plan:
* Add billing_periods column to coupons and coupon_codes tables (this change)
* After one release, remove all references to the old duration column,
replacing with references to billing_periods. At this point, we can also
change the defult promotional coupon to never expire and migrate over
values from the old duration column.
* After another release, drop the duration column.
Change-Id: I374e8dc9fab9f81b4a5bc681771955662d4c007a
