The maximum content count of a broadcast varies depending on its
encoding, so we can’t simply validate it against a schema. This commit
moves to using the validation from `notifications-utils`, and raising a
custom error response.
We don’t support these methods at the moment. Instead we were just
ignoring the `msgType` field, so issuing one of these commands would
cause a new alert to be broadcast 🙃
We might want to support `Cancel` in the future, but for now let’s
reject anything that isn’t `Alert` (CAP terminology for the initial
broadcast).
