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()
```

app/assets/javascripts/admin/templates/components/site-settings/category-list.hbs

@@ -1,3 +1,3 @@
-{{category-selector categories=selectedCategories blacklist=selectedCategories}}
+{{category-selector categories=selectedCategories}}
 <div class='desc'>{{{unbound setting.description}}}</div>
 {{setting-validation-message message=validationMessage}}

app/assets/javascripts/admin/templates/web-hooks-show.hbs

@@ -48,7 +48,7 @@
     <div class='filters'>
       <div>
         <label>{{d-icon 'circle' class='tracking'}}{{i18n 'admin.web_hooks.categories_filter'}}</label>
-        {{category-selector categories=model.categories blacklist=model.categories}}
+        {{category-selector categories=model.categories}}
         <div class="instructions">{{i18n 'admin.web_hooks.categories_filter_instructions'}}</div>
       </div>
       <div>