Remove conversation URI (#11423)

It is not part of ActivityPub and will free up a lot of space

db/post_migrate/20190519130537_remove_boosts_widening_audience.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class RemoveBoostsWideningAudience < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 

db/post_migrate/20190728084117_remove_uri_from_conversations.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class RemoveUriFromConversations < ActiveRecord::Migration[5.2]
+  def up
+    safety_assured { remove_column :conversations, :uri, :string }
+  end
+
+  def down
+    add_column :conversations, :uri, :string
+    add_index :conversations, :uri, unique: true
+  end
+end