See "[EPIC] Audio metadata update"
- Ensure we parse received activities with a Json-LD parser, and implement all the cleaning layers to deal with expanded jsonld payloads
- Use this new logic on all federation-related serializers
- Implement logic to dereference jsonld objects, when needed (it's not used anywhere in the code for now)
- Fix a bug that caused a crash with transient activities
- Use cached jsonld contexts to ensure we don't send absurd numbers of HTTP requests on the network