Use Ecto.Schema in ActivityPub / Transmogrifier
Right now, the validation of incoming activities mostly involves matching on the type and some additional data, then "fixing up" (i.e. turning into our internal representation) the data. This is somewhat dangerous and can lead to breakage if the incoming data isn't quite what we expect, as recently happened with the honk activities.
It would be nice to do at least some basic validation. We should be able to use the Ecto.Schema framework for this.