DRY up UrlsController with a before_filter

In a few actions we had repeated the same finding of the Url instance
from the id param and setting it as an instance variable in this
controller. To avoid repetition, I'm moving this to a before_filter.
This will make the code more maintainable.

Change-Id: Ibf14eaef4513d4da1f0ce239207bb15e6af3196f