Currently, private / direct posts via OStatus from AP compatible instance will be dropped due to failing to fetch AP version.
So this fallbacks to OStatus handling:
* when failed to fetch ActivityPub version
* when status is neither :public nor :unlisted
* Wrap methods of ProcessFeedService::ProcessEntry in classes
This is a change same with 425acecfdb, except
that it has the following changes:
* Revert irrelevant change in find_or_create_conversation
* Fix error handling for RemoteActivity
* Introduce Ostatus name space
