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/general.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class Ostatus::Activity::General < Ostatus::Activity::Base
+  def specialize
+    special_class&.new(@xml, @account)
+  end
+
+  private
+
+  def special_class
+    case verb
+    when :post
+      Ostatus::Activity::Post
+    when :share
+      Ostatus::Activity::Share
+    when :delete
+      Ostatus::Activity::Deletion
+    end
+  end
+end