UX: replace FA5 compress/expand icons

Adds a new SVG sprite file that contains non-FontAwesome 5 icons to our subset.

Adds the FontAwesome 4.7.0 icons for expand / collapse.

app/assets/javascripts/admin/controllers/admin-customize-themes-edit.js.es6

@@ -154,7 +154,7 @@ export default Ember.Controller.extend({
 
   @computed("maximized")
   maximizeIcon(maximized) {
-    return maximized ? "compress" : "expand";
+    return maximized ? "discourse-compress" : "discourse-expand";
   },
 
   @computed("model.isSaving")