Introduce Ostatus name space (#4164)

* 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

app/controllers/stream_entries_controller.rb

@@ -19,7 +19,7 @@ class StreamEntriesController < ApplicationController
       end
 
       format.atom do
-        render xml: AtomSerializer.render(AtomSerializer.new.entry(@stream_entry, true))
+        render xml: Ostatus::AtomSerializer.render(Ostatus::AtomSerializer.new.entry(@stream_entry, true))
       end
     end
   end