Add confirmation banner when cancelling user invites

This shows the green banner with a tick when cancelling a user's
invitation to a service or organisation. The accessibility audit noted
that 'When cancelling an invite a new page loads, however, there is no
immediate indication that the invite has been cancelled.'

In order to display the invited user's email address as part of the
flash message, this adds new methods to the api clients for invites to get
a single invite.

app/notify_client/invite_api_client.py

@@ -37,6 +37,11 @@ class InviteApiClient(NotifyAdminAPIClient):
             '/service/{}/invite'.format(service_id)
         )['data']
 
+    def get_invited_user(self, service_id, invited_user_id):
+        return self.get(
+            f'/service/{service_id}/invite/{invited_user_id}'
+        )['data']
+
     def get_count_of_invites_with_permission(self, service_id, permission):
         if permission not in roles.keys():
             raise TypeError('{} is not a valid permission'.format(permission))