Truncate long client URNs during CLS JS URL generation.

More acronyms ASAP.

This hasn't come up yet because the only live client had a URN that was shorter than Heroku's 30 character subdomain length limit.

This helper's existence probably exposes a few missing features. I think widgets aren't quite dynamical enough to calculate this URL, so this is loaded on every page via a layout? Also, there's no real good method for determining a herokuapp URL within a constellation.  Neither of those things I can address right now.