Add migration to default Mute#hide_notifications? to true

db/migrate/20170914032032_default_existing_mutes_to_hiding_notifications.rb

@@ -0,0 +1,8 @@
+class DefaultExistingMutesToHidingNotifications < ActiveRecord::Migration[5.1]
+  def up
+    t.change_column_default :mutes, :hide_notifications, from: false, to: true
+
+    # Unfortunately if this is applied sometime after the one to add the table we lose some data, so this is irreversible.
+    Mutes.update_all(hide_notifications: true)
+  end
+end