See #689 (closed)

This MR:

• 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