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-delete-flag-dropdown.js.es6

@@ -0,0 +1,77 @@
+import DropdownSelectBox from "select-kit/components/dropdown-select-box";
+import computed from "ember-addons/ember-computed-decorators";
+
+export default DropdownSelectBox.extend({
+  classNames: ["delete-flag", "admin-delete-flag-dropdown"],
+  adminTools: Ember.inject.service(),
+  nameProperty: "label",
+  headerIcon: "trash-o",
+
+  computeHeaderContent() {
+    let content = this.baseHeaderComputedContent();
+    content.name = I18n.t("admin.flags.delete");
+    return content;
+  },
+
+  @computed("adminTools", "post.user")
+  spammerDetails(adminTools, user) {
+    return adminTools.spammerDetails(user);
+  },
+
+  canDeleteSpammer: Ember.computed.and("spammerDetails.canDelete", "post.flaggedForSpam"),
+
+  computeContent() {
+    const content = [];
+    const canDeleteSpammer = this.get("canDeleteSpammer");
+
+    content.push({
+      icon: "external-link",
+      id: "delete-defer",
+      action: () => this.send("deletePostDeferFlag"),
+      label: I18n.t("admin.flags.delete_post_defer_flag"),
+      description:  I18n.t("admin.flags.delete_post_defer_flag_title"),
+    });
+
+    content.push({
+      icon: "thumbs-o-up",
+      id: "delete-agree",
+      action: () => this.send("deletePostAgreeFlag"),
+      label: I18n.t("admin.flags.delete_post_agree_flag"),
+      description:  I18n.t("admin.flags.delete_post_agree_flag_title"),
+    });
+
+    if (canDeleteSpammer) {
+      content.push({
+        title:  I18n.t("admin.flags.delete_post_agree_flag_title"),
+        icon: "exclamation-triangle",
+        id: "delete-spammer",
+        action: () => this.send("deleteSpammer"),
+        label: I18n.t("admin.flags.delete_spammer")
+      });
+    }
+
+    return content;
+  },
+
+  mutateValue(value) {
+    const computedContentItem = this.get("computedContent").findBy("value", value);
+    Ember.get(computedContentItem, "originalContent.action")();
+  },
+
+  actions: {
+    deleteSpammer() {
+      let spammerDetails = this.get("spammerDetails");
+      this.attrs.removeAfter(spammerDetails.deleteUser());
+    },
+
+    deletePostDeferFlag() {
+      let flaggedPost = this.get('post');
+      this.attrs.removeAfter(flaggedPost.deferFlags(true));
+    },
+
+    deletePostAgreeFlag() {
+      let flaggedPost = this.get('post');
+      this.attrs.removeAfter(flaggedPost.agreeFlags('delete'));
+    }
+  }
+});