Use closest `.alert` for alert dismissal

Created by: hnrch02

Currently we do this to find the element which should be removed if the dismiss button is clicked. Wouldn't it make more sense to use something like .closest('.alert') to traverse higher up the tree instead of just one level? This would allow for the dismiss button to be nested somewhere deeper than as a direct descendant of the alert. (Think alerts which span the full width of the page and contain a .container.)

/cc @fat @mdo