feedback (see commit description for details)

* fill blank space when no theme is selected
* animate row's height in themes/components list when selecting, and hide children list
* show warning when you move to a different page and have unsaved changes
* refactor `adminCustomizeThemes.show` controller
* allow collapsing/expanding children lists
* fix a bug when adding components to a theme (changed the way it works slightly)
* a bunch of other minor things

test/javascripts/admin/components/themes-list-item-test.js.es6

@@ -62,7 +62,7 @@ componentTest("with children", {
   beforeEach() {
     this.set(
       "theme",
-      Theme.create({ name: "Test", childThemes: childrenList })
+      Theme.create({ name: "Test", childThemes: childrenList, default: true })
     );
   },