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

app/models/conversation.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: conversations
+#
+#  id         :integer          not null, primary key
+#  uri        :string
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class Conversation < ApplicationRecord
+  validates :uri, uniqueness: true
+
+  has_many :statuses
+
+  def local?
+    uri.nil?
+  end
+end