in the NotifyAdminAPIClient, which all api traffic goes through, return
403 for any stateful requests (post, put and delete), if the following
criteria have been met:
* a current_service is set
(this prevents checks being carried out on non-service related
updates, eg editing user details)
* the service is not active
* the current user is not a platform admin
so platform admins can still update anything.
Note: Without any specific error handling, the user will see a generic
403 page. This is fine, probably - it's a relatively niche case that
you'll be editing a service you can't get to anyway
