display_settings: Change success/failure feedback interface.

This replaces the cumbersome system we had for giving users feedback
on settings state changes in the display settings UI.

We expect this new system to be what we will attempt to migrate other
settings widgets to match over the coming weeks and months.  It also
provides the opportunity to significant refactor away a lot of the
code duplication in settings_display.js.

Thanks to Brock Whittaker for redoing the styling and improving the
code simplicity.

Fixes #7622.

static/js/loading.js

@@ -46,7 +46,7 @@ exports.make_indicator = function (outer_container, opts) {
     // These width calculations are tied to the spinner width and
     // margins defined via CSS
     container.css({width: 38 + text_width,
-                   height: 38});
+                   height: 0});
 
     outer_container.data("destroying", false);
 };