Add model properties for template and folder IDs
Add model properties for template and folder IDs

We’re going to need a way to find out if, for a given ID:
1. its a template or a folder
2. if it belongs to a given service

To do 2., we can make a set of all the IDs, to see if a given ID is a
member of that set.

To do 1. we can make two sets, one for templates and one for folders.

The natural home for these sets is on the service model.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin