Provide an account_code to Entity redirect URL

As mentioned in #1630772: Push account updates to Recurly contact information, we currently have issues where we don't have a way to link to account management pages from within the Recurly dunning e-mails. Because the tokens available in the Recurly templates only include Account Code ({{account_code}}) and the account code does not necessarily line up 1-to-1 with the Drupal ID, we need some kind of Drupal URL that will redirect to the user's account on the Drupal site. Something like http://example.com/manage-subscription/{{account_code}}, which would redirect to http://example.com/user/[uid]/subscription. Considering the point of the Recurly built-in pages is to avoid exposing Recurly to the end-user, we should avoid the use of the word Recurly in the external-facing URL.