Revert "Remove conversation URI (#11423)" (#11424)

This reverts commit 75f7f9930e.

app/models/conversation.rb

@@ -4,10 +4,17 @@
 # Table name: conversations
 #
 #  id         :bigint(8)        not null, primary key
+#  uri        :string
 #  created_at :datetime         not null
 #  updated_at :datetime         not null
 #
 
 class Conversation < ApplicationRecord
+  validates :uri, uniqueness: true, if: :uri?
+
   has_many :statuses
+
+  def local?
+    uri.nil?
+  end
 end