A previous commit added a helper function for sending JSON data back to
the client.
This commit makes use of it for homogenizing the current implementation.
It also renames the existing helper message to send JSON errors to
starts with "send" because the new helper starts with it and they
helpers are clearer with their name starting with it.
Change-Id: I53ee0b4ca33d677a8ccd366c9ba6d73f4f472247
* 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
