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).
This commit makes the existing endpoint also accept CAP XML, should the
appropriate `Content-Type` header be set.
It uses the translation code we added in a previous commit to convert
the CAP to a dict. We can then validate that dict against with the JSON
schema to ensure it’s something we can work with.
