Hook up URL-based resource look-up to ActivityPub (#4589)

app/services/activitypub/fetch_remote_account_service.rb

@@ -5,8 +5,8 @@ class ActivityPub::FetchRemoteAccountService < BaseService
 
   # Should be called when uri has already been checked for locality
   # Does a WebFinger roundtrip on each call
-  def call(uri)
-    @json = fetch_resource(uri)
+  def call(uri, prefetched_json = nil)
+    @json = body_to_json(prefetched_json) || fetch_resource(uri)
 
     return unless supported_context? && expected_type?