Assets controller fixes

* Replace usages of Play.current with Play.maybeApplication. This
  ensures that at very least templates using assets reverse routes can
  be rendered without a running application.
* Used Option instead of exceptions to handle NotFound
* Fixed resource leak in digest cache
* Use Global bad request handling
* Let Play handle errors
Merge pull request #3122 from jroper/assets-no-play-current

Don't use Play.current anywhere in Assets