Added the random string reference to the letter

- uses the reference field on the notifications table to store a 16char random string used to cross reference DVLA letters back to the notification
- used as letter barcode does not have space for a UUID notification id

Depends on https://github.com/alphagov/notifications-utils/pull/149

Renamed the numeric_id to notification_reference in utils and changed validation rules to match this

Note also the persist_notification method set "reference" to be "client_reference" which is confusing and they are different things, so fixed this too.

app/__init__.py

@@ -1,4 +1,6 @@
 import os
+import random
+import string
 import uuid
 
 from flask import Flask, _request_ctx_stack
@@ -204,6 +206,10 @@ def create_uuid():
     return str(uuid.uuid4())
 
 
+def create_random_identifier():
+    return ''.join(random.choice(string.ascii_letters + string.digits) for _ in range(16))
+
+
 def process_user_agent(user_agent_string):
     if user_agent_string and user_agent_string.lower().startswith("notify"):
         components = user_agent_string.split("/")