Allow platform admins to cancel broadcasts.

Also update error message for when someone does not have permissions.

The message referenced approving broadcasts specifically, whereas
people would also see it if they try to cancel or reject
broadcast without permission.

Why we allow platform admins to cancel broadcasts:
we do this so they can react quickly if a broadcast was
approved by accident.

app/broadcast_message/rest.py

@@ -43,10 +43,12 @@ def _parse_nullable_datetime(dt):
 
 def _update_broadcast_message(broadcast_message, new_status, updating_user):
     if updating_user not in broadcast_message.service.users:
-        raise InvalidRequest(
-            f'User {updating_user.id} cannot approve broadcast_message {broadcast_message.id} from other service',
-            status_code=400
-        )
+        #  we allow platform admins to cancel broadcasts
+        if not (new_status == BroadcastStatusType.CANCELLED and updating_user.platform_admin):
+            raise InvalidRequest(
+                f'User {updating_user.id} cannot update broadcast_message {broadcast_message.id} from other service',
+                status_code=400
+            )
 
     if new_status not in BroadcastStatusType.ALLOWED_STATUS_TRANSITIONS[broadcast_message.status]:
         raise InvalidRequest(