Speed up GET services endpoints

Add additional relationships to exclude in the ServiceSchema metaclass.
Marshmallow by default lazily loads relationships when dumping, so any
relationships we know we won't need, we can exclude and avoid a DB call.
Lots of tables are linked to services, so it loads a lot of tables.
So don't load statistics tables, since they're clearly not needed.

We *do* however want to return the users for the service - they're used
in a few places. If we're returning all services, then we don't want to
make separate queries for these users, so we modify the services_dao
queries to load users the first time round. This should speed up all
GET queries to the services endpoints, most notably pages that get many
services (platform_admin, choose service, login)

app/schemas.py

@@ -106,7 +106,15 @@ class ServiceSchema(BaseSchema):
 
     class Meta:
         model = models.Service
-        exclude = ("updated_at", "created_at", "api_keys", "templates", "jobs", 'old_id')
+        exclude = ('updated_at',
+                   'created_at',
+                   'api_keys',
+                   'templates',
+                   'jobs',
+                   'old_id',
+                   'template_statistics',
+                   'service_provider_stats',
+                   'service_notification_stats')
         strict = True
 
     @validates('sms_sender')