add inbound sms api

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

app/models.py

@@ -1175,6 +1175,18 @@ class InboundSms(db.Model):
     def content(self, content):
         self._content = encryption.encrypt(content)
 
+    def serialize(self):
+        return {
+            'id': str(self.id),
+            'created_at': self.created_at.isoformat(),
+            'service_id': str(self.service_id),
+            'notify_number': self.notify_number,
+            'user_number': self.user_number,
+            'content': self.content,
+            'provider_date': self.provider_date and self.provider_date.isoformat(),
+            'provider_reference': self.provider_reference
+        }
+
 
 class LetterRate(db.Model):
     __tablename__ = 'letter_rates'