purge site/user css/js on edit

This is a bit of a "hack" because it involves calling a protected function in OutputPage.  However, that function has about 200 lines of code and re-implemeting it involves copying about 50 lines to a helper... :/  

I think it's cleaner in this case to just to make it public and re-use the logic.   However, if there are strong objections to this approach I am willing to refactor.

@Wikia/platform-team