[UX] Add a button for clearing css/js cache and theme registry to the appearence page

We had a forum post that highlighted how people new to Backdrop get frustrated when they are not able to see their theme changes right away. Even when someone mentioned looking for theme update/refresh options they were unable to find the "Flush CSS/JS cache" link.

I would like to propose that we add a button to the top of the Appearance page for clearing the CSS/JS cache.

-- PR: https://github.com/backdrop/backdrop/pull/3054