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/deletion.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class Ostatus::Activity::Deletion < Ostatus::Activity::Base
+  def perform
+    Rails.logger.debug "Deleting remote status #{id}"
+    status = Status.find_by(uri: id, account: @account)
+
+    if status.nil?
+      redis.setex("delete_upon_arrival:#{@account.id}:#{id}", 6 * 3_600, id)
+    else
+      RemoveStatusService.new.call(status)
+    end
+  end
+end