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/services/concerns/stream_entry_renderer.rb

@@ -2,6 +2,6 @@
 
 module StreamEntryRenderer
   def stream_entry_to_xml(stream_entry)
-    AtomSerializer.render(AtomSerializer.new.entry(stream_entry, true))
+    Ostatus::AtomSerializer.render(Ostatus::AtomSerializer.new.entry(stream_entry, true))
   end
 end