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/lib/ostatus/activity/remote.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class Ostatus::Activity::Remote < Ostatus::Activity::Base
+  def perform
+    find_status(id) || FetchRemoteStatusService.new.call(url)
+  end
+end