Add conversation model, <ostatus:conversation /> (#3016)

* Add <ostatus:conversation /> tag to Atom input/output

Only uses ref attribute (not href) because href would be
the alternate link that's always included also.

Creates new conversation for every non-reply status. Carries
over conversation for every reply. Keeps remote URIs verbatim,
generates local URIs on the fly like the rest of them.

* Fix conversation migration

* More spec coverage for status before_create

* Prevent n+1 query when generating Atom with the new conversations

* Improve code style

* Remove redundant local variable

db/migrate/20170507000211_add_conversation_id_to_statuses.rb

@@ -0,0 +1,6 @@
+class AddConversationIdToStatuses < ActiveRecord::Migration[5.0]
+  def change
+    add_column :statuses, :conversation_id, :bigint, null: true, default: nil
+    add_index :statuses, :conversation_id
+  end
+end