Refactor high volume into serialised service model

Just looks a bit tidier and less repetitive.

I’ve only done this for the serialised service because:
- we’re only checking this in places where we’re already using the
  serialised service
- if we want to check this elsewhere there’s a good chance that new code
  should be using the serialised service, since it’ll itself be doing
  some kind of performance optimisation

app/serialised_models.py

@@ -3,6 +3,7 @@ from functools import partial
 from threading import RLock
 
 import cachetools
+from flask import current_app
 from notifications_utils.clients.redis import RequestCache
 from notifications_utils.serialised_model import (
     SerialisedModel,
@@ -106,6 +107,10 @@ class SerialisedService(SerialisedModel):
     def api_keys(self):
         return SerialisedAPIKeyCollection.from_service_id(self.id)
 
+    @property
+    def high_volume(self):
+        return self.id in current_app.config['HIGH_VOLUME_SERVICE']
+
 
 class SerialisedAPIKey(SerialisedModel):
     ALLOWED_PROPERTIES = {