refs: https://github.com/TryGhost/Toolbox/issues/245
- Added a serializer called default to the canary API
- Ideally, this would be part of the shared framework, but this would change v2/v3 and we're about to get rid of them
- Therefore, we change just canary for now, and we can refactor again later.
- Added wiring to handler that uses the default serializer, if there is a default, and isn't an explicit serializer for the endpoint
- Removed the invites serializer, so that one endpoint now uses the default
Note: previous commits have added explicit serializers to every endpoint, this is the first step towards paring
that back so that we have less serializers overall, not more!
