make inbound sms honour 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

tests/app/db.py

@@ -1,3 +1,4 @@
+import random
 import uuid
 from datetime import datetime, date
 
@@ -318,10 +319,18 @@ def create_inbound_sms(
         provider="mmg",
         created_at=None
 ):
+    if not service.inbound_number:
+        create_inbound_number(
+            # create random inbound number
+            notify_number or '07{:09}'.format(random.randint(0, 1e9 - 1)),
+            provider=provider,
+            service_id=service.id
+        )
+
     inbound = InboundSms(
         service=service,
         created_at=created_at or datetime.utcnow(),
-        notify_number=notify_number or service.get_default_sms_sender(),
+        notify_number=service.get_inbound_number(),
         user_number=user_number,
         provider_date=provider_date or datetime.utcnow(),
         provider_reference=provider_reference or 'foo',