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/inbound_sms/rest.py

@@ -0,0 +1,41 @@
+from flask import (
+    Blueprint,
+    jsonify,
+    request
+)
+from notifications_utils.recipients import normalise_phone_number
+
+from app.dao.inbound_sms_dao import dao_get_inbound_sms_for_service, dao_count_inbound_sms_for_service
+from app.errors import register_errors
+
+inbound_sms = Blueprint(
+    'inbound_sms',
+    __name__,
+    url_prefix='/service/<service_id>/inbound-sms'
+)
+
+register_errors(inbound_sms)
+
+
+@inbound_sms.route('')
+def get_inbound_sms_for_service(service_id):
+    limit = request.args.get('limit')
+    user_number = request.args.get('user_number')
+
+    if user_number:
+        user_number = normalise_phone_number(user_number)
+
+    results = dao_get_inbound_sms_for_service(service_id, limit, user_number)
+
+    return jsonify(data=[row.serialize() for row in results])
+
+
+@inbound_sms.route('/summary')
+def get_inbound_sms_summary_for_service(service_id):
+    count = dao_count_inbound_sms_for_service(service_id)
+    most_recent = dao_get_inbound_sms_for_service(service_id, limit=1)
+
+    return jsonify(
+        count=count,
+        most_recent=most_recent[0].created_at.isoformat() if most_recent else None
+    )