DRY up UrlsController with a before_filter
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

diffux/diffux,kalw/diffux,diffux/diffux,diffux/diffux,kalw/diffux,kalw/diffux