Add explicit primary key declaration for changeset subscriptions

This table has a composite primary key and although we don't really
rely on that at all rails 5.0 warns about itif you don't explcitly
declare it in order to trigger the CPK extension.

app/models/diary_entry_subscription.rb

@@ -1,4 +1,6 @@
 class DiaryEntrySubscription < ActiveRecord::Base
+  self.primary_keys = "user_id", "diary_entry_id"
+
   belongs_to :user
   belongs_to :diary_entry
 end