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/post.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class Ostatus::Activity::Post < Ostatus::Activity::Creation
+  def perform
+    status, just_created = super
+
+    if just_created
+      status.mentions.includes(:account).each do |mention|
+        mentioned_account = mention.account
+        next unless mentioned_account.local?
+        NotifyService.new.call(mentioned_account, mention)
+      end
+    end
+
+    status
+  end
+
+  private
+
+  def reblog
+    nil
+  end
+end