Make user API client return JSON, not a model

The data flow of other bits of our application looks like this:
```
                         API (returns JSON)
                                  ⬇
          API client (returns a built in type, usually `dict`)
                                  ⬇
          Model (returns an instance, eg of type `Service`)
                                  ⬇
                         View (returns HTML)
```
The user API client was architected weirdly, in that it returned a model
directly, like this:

```
                         API (returns JSON)
                                  ⬇
    API client (returns a model, of type `User`, `InvitedUser`, etc)
                                  ⬇
                         View (returns HTML)
```

This mixing of different layers of the application is bad because it
makes it hard to write model code that doesn’t have circular
dependencies. As our application gets more complicated we will be
relying more on models to manage this complexity, so we should make it
easy, not hard to write them.

It also means that most of our mocking was of the User model, not just
the underlying JSON. So it would have been easy to introduce subtle bugs
to the user model, because it wasn’t being comprehensively tested. A lot
of the changed lines of code in this commit mean changing the tests to
mock only the JSON, which means that the model layer gets implicitly
tested.

For those reasons this commit changes the user API client to return
JSON, not an instance of `User` or other models.

app/models/service.py

@@ -4,6 +4,7 @@ from werkzeug.utils import cached_property
 
 from app.models import JSONModel
 from app.models.organisation import Organisation
+from app.models.user import InvitedUsers, User, Users
 from app.notify_client.api_key_api_client import api_key_api_client
 from app.notify_client.billing_api_client import billing_api_client
 from app.notify_client.email_branding_client import email_branding_client
@@ -16,7 +17,6 @@ from app.notify_client.service_api_client import service_api_client
 from app.notify_client.template_folder_api_client import (
     template_folder_api_client,
 )
-from app.notify_client.user_api_client import user_api_client
 from app.utils import get_default_sms_sender
 
 
@@ -60,6 +60,10 @@ class Service(JSONModel):
         if 'permissions' not in self._dict:
             self.permissions = {'email', 'sms', 'letter'}
 
+    @classmethod
+    def from_id(cls, service_id):
+        return cls(service_api_client.get_service(service_id)['data'])
+
     def update(self, **kwargs):
         return service_api_client.update_service(self.id, **kwargs)
 
@@ -99,11 +103,11 @@ class Service(JSONModel):
 
     @cached_property
     def invited_users(self):
-        return invite_api_client.get_invites_for_service(service_id=self.id)
+        return InvitedUsers(self.id)
 
     @cached_property
     def active_users(self):
-        return user_api_client.get_users_for_service(service_id=self.id)
+        return Users(self.id)
 
     @cached_property
     def team_members(self):
@@ -114,16 +118,12 @@ class Service(JSONModel):
 
     @cached_property
     def has_team_members(self):
-        return (
-            user_api_client.get_count_of_users_with_permission(
-                self.id, 'manage_service'
-            ) + invite_api_client.get_count_of_invites_with_permission(
-                self.id, 'manage_service'
-            )
-        ) > 1
+        return len([
+            user for user in self.team_members
+            if user.has_permission_for_service(self.id, 'manage_service')
+        ]) > 1
 
     def cancel_invite(self, invited_user_id):
-
         if str(invited_user_id) not in {user.id for user in self.invited_users}:
             abort(404)
 
@@ -137,7 +137,7 @@ class Service(JSONModel):
         if str(user_id) not in {user.id for user in self.active_users}:
             abort(404)
 
-        return user_api_client.get_user(user_id)
+        return User.from_id(user_id)
 
     @cached_property
     def all_templates(self):