Send and accept only spec-compliant ActivityPub headers

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According to the ActivityPub spec:

The HTTP GET method may be dereferenced against an object's id property to retrieve the activity. Servers MAY use HTTP content negotiation as defined in [RFC7231] to select the type of data to return in response to a request, but MUST present the ActivityStreams object representation in response to application/ld+json; profile="https://www.w3.org/ns/activitystreams", and SHOULD also present the ActivityStreams representation in response to application/activity+json as well. The client MUST specify an Accept header with the application/ld+json; profile="https://www.w3.org/ns/activitystreams" media type in order to retrieve the activity.

This PR standardizes the use of Accept and Content-Type headers in our AP implementation. Namely, for best compatibility, we now always send Content-Type: application/ld+json; profile="https://www.w3.org/ns/activitystreams", but accept either of the headers if they're specified in Accept.

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Tests against Mastodon all work, so I'm going to merge this in.