also, it should default to last 7 days, not last 6 days. also change
count_inbound_sms to have the days passed in, so that it's more
explicit at the endpoint that we only return 7 days regardless of your
service's data retention
code inspired by the delete notification code, but with some clean up
since we don't deal with different types etc, and only need to run the
query for services with inbound numbers
also, update tests.app.db.create_inbound_sms to create inbound numbers
and assign them to services to ensure the test db is always accurate
and reflects real world usage
two endpoints:
* get all inbound sms for a service (you can limit to the X most
recent, or filter by user's phone number [which will be normalised])
* get a summary of inbound sms for a service - returns the count of
inbound sms in the database, and the date that the most recent was
sent
