[UX] Add "Update available" indicator to the modules list.

This one is exactly the same request as https://www.drupal.org/node/197486.

Having an indicator on the modules list page would remove a duplicate interface that lists all modules, and be a more intuitive place for site builders to find available update information. Additionally, having the indicator in a place site builders visit all the time might also encourage updating more often, creating more secure sites for all.