Simply ignore unknown repeater types

This is an attempt at a surgical change to the receiverwrapper so it does not crash on unfamiliar `Repeater` types. This is consistent with the idea that the `repeater_types` are an extensible registry and the database may have registered repeaters from an app that is not currently installed.

Also, the API allows any repeater to be POSTed, as that is the simplest and most flexible no-crashing option.

This fixes an issue where someone did, in fact, POST an unknown repeater type, and it was causing some crashes.