Introduces select-kit

* renames `select-box-kit` into `select-kit`
* introduces `single-select` and `multi-select` as base components
* introduces {{search-advanced-category-chooser}} as a better component for selecting category in advanced search
* improves events handling in select-kit
* recreates color selection inputs using {{multi-select}} and a custom {{selected-color}} component
* replaces category-selector by a component using select-kit and based on multi-select
* improves positioning of wrapper
* removes the need for offscreen, and instead use `select-kit-header` as a base focus point for all select-kit based components
* introduces a formal plugin api for select-kit based components
* introduces a formal pattern for loading and updating select-kit based components:

```
computeValue()
computeContent()
mutateValue()
```

test/javascripts/acceptance/topic-notifications-button-test.js.es6

@@ -28,12 +28,12 @@ QUnit.test("Updating topic notification level", assert => {
     );
   });
 
-  expandSelectBoxKit(notificationOptions);
-  selectBoxKitSelectRow("3", { selector: notificationOptions});
+  expandSelectKit(notificationOptions);
+  selectKitSelectRow("3", { selector: notificationOptions});
 
   andThen(() => {
     assert.equal(
-      selectBox(notificationOptions).selectedRow.name(),
+      selectKit(notificationOptions).selectedRow.name(),
       "watching",
       "it should display the right notification level"
     );