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/select-kit/components/admin-group-selector.js.es6

@@ -0,0 +1,51 @@
+import MultiSelectComponent from "select-kit/components/multi-select";
+const { makeArray } = Ember;
+
+export default MultiSelectComponent.extend({
+  pluginApiIdentifiers: ["admin-group-selector"],
+  classNames: "admin-group-selector",
+  selected: null,
+  available: null,
+  allowAny: false,
+
+  computeValues() {
+    return makeArray(this.get("selected"))
+      .map(s => this.valueForContentItem(s));
+  },
+
+  computeContent() {
+    return makeArray(this.get("available"));
+  },
+
+  computeContentItem(contentItem, name) {
+    let computedContent = this.baseComputedContentItem(contentItem, name);
+    computedContent.locked = contentItem.automatic;
+    return computedContent;
+  },
+
+  mutateValues(values) {
+    if (values.length > this.get("selected").length) {
+      const newValues = values
+        .filter(v => !this.get("selected")
+        .map(s => this.valueForContentItem(s))
+        .includes(v));
+
+      newValues.forEach(value => {
+        const actionContext = this.get("available")
+          .findBy(this.get("valueAttribute"), parseInt(value, 10));
+
+        this.triggerAction({ action: "groupAdded", actionContext });
+      });
+    } else if (values.length < this.get("selected").length) {
+      const selected = this.get("selected")
+        .filter(s => !values.includes(this.valueForContentItem(s)));
+
+      selected.forEach(s => {
+        this.triggerAction({
+          action: "groupRemoved",
+          actionContext: this.valueForContentItem(s)
+        });
+      });
+    }
+  }
+});