Refactor filtering out accepted invites to client

None of our model or view layer code should need to know about accepted
invites. We don’t use them anywhere because once an invite is accepted
that person is now a user.

Putting this logic in the client means that:
- none of the code calling the client needs to care about accepted
  invites
- it’s easier to (if we want) update the API code to not return accepted
  invites

app/notify_client/invite_api_client.py

@@ -28,11 +28,16 @@ class InviteApiClient(NotifyAdminAPIClient):
         return InvitedUser(**resp['data'])
 
     def get_invites_for_service(self, service_id):
-        endpoint = '/service/{}/invite'.format(service_id)
-        resp = self.get(endpoint)
-        invites = resp['data']
-        invited_users = self._get_invited_users(invites)
-        return invited_users
+        return [
+            InvitedUser(**invite)
+            for invite in self._get_invites_for_service(service_id)
+            if invite['status'] != 'accepted'
+        ]
+
+    def _get_invites_for_service(self, service_id):
+        return self.get(
+            '/service/{}/invite'.format(service_id)
+        )['data']
 
     def check_token(self, token):
         resp = self.get(url='/invite/service/{}'.format(token))
@@ -51,12 +56,5 @@ class InviteApiClient(NotifyAdminAPIClient):
         self.post(url='/service/{0}/invite/{1}'.format(service_id, invited_user_id),
                   data=data)
 
-    def _get_invited_users(self, invites):
-        invited_users = []
-        for invite in invites:
-            invited_user = InvitedUser(**invite)
-            invited_users.append(invited_user)
-        return invited_users
-
 
 invite_api_client = InviteApiClient()