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 425acecfdb15093a265b191120fb2d4e4c4135c4, except
that it has the following changes:

* Revert irrelevant change in find_or_create_conversation
* Fix error handling for RemoteActivity

* Introduce Ostatus name space