[gh-497] Create localized 'Purge Varnish' for Site Admin

I have a scenario where reordering a view with Draggable Views is not updating the page for anonymous users because varnish is not being cleared.

Rather than figure out a hook just for this use case, I'm fine with telling people to 'go hit the button' but we don't have one. Purge Varnish under performance wipes out varnish system wide.

We do have site-level varnish clearing when a page is updated via use of varnish_purge_all_pages()

I think the current "Purge Varnish" should be default site only and a localized one is needed.