Improve ActivityPub/OStatus compatibility (#4632)

*Note: OStatus URIs are invalid for ActivityPub. But we have them for
as long as we want to keep old OStatus-sourced content and as long as
we remain OStatus-compatible.*

- In Announce handling, if object URI is not a URL, fallback to object URL
- Do not use specialized ThreadResolveWorker, rely on generalized handling
- When serializing notes, if parent's URI is not a URL, use parent's URL

spec/workers/activitypub/thread_resolve_worker_spec.rb

@@ -1,16 +0,0 @@
-require 'rails_helper'
-
-describe ActivityPub::ThreadResolveWorker do
-  subject { described_class.new }
-
-  let(:status) { Fabricate(:status) }
-  let(:parent) { Fabricate(:status) }
-
-  describe '#perform' do
-    it 'gets parent from ActivityPub::FetchRemoteStatusService and glues them together' do
-      allow(ActivityPub::FetchRemoteStatusService).to receive(:new).and_return(double(:service, call: parent))
-      subject.perform(status.id, 'http://example.com/123')
-      expect(status.reload.in_reply_to_id).to eq parent.id
-    end
-  end
-end