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/multi-select/multi-select-header.js.es6

@@ -0,0 +1,32 @@
+import { on } from "ember-addons/ember-computed-decorators";
+import computed from "ember-addons/ember-computed-decorators";
+import SelectKitHeaderComponent from "select-kit/components/select-kit/select-kit-header";
+
+export default SelectKitHeaderComponent.extend({
+  attributeBindings: ["names:data-name"],
+  classNames: "multi-select-header",
+  layoutName: "select-kit/templates/components/multi-select/multi-select-header",
+  selectedNameComponent: Ember.computed.alias("options.selectedNameComponent"),
+
+  @on("didRender")
+  _positionFilter() {
+    if (this.get("shouldDisplayFilter") === false) { return; }
+
+    const $filter = this.$(".filter");
+    $filter.width(0);
+
+    const leftHeaderOffset = this.$().offset().left;
+    const leftFilterOffset = $filter.offset().left;
+    const offset = leftFilterOffset - leftHeaderOffset;
+    const width = this.$().outerWidth(false);
+    const availableSpace = width - offset;
+    const $choices = $filter.parent(".choices");
+    const parentRightPadding = parseInt($choices.css("padding-right") , 10);
+    $filter.width(availableSpace - parentRightPadding * 4);
+  },
+
+  @computed("computedContent.selectedComputedContents.[]")
+  names(selectedComputedContents) {
+    return Ember.makeArray(selectedComputedContents).map(sc => sc.name).join(",");
+  }
+});

app/assets/javascripts/select-kit/components/multi-select/selected-category.js.es6

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+import SelectedNameComponent from "select-kit/components/multi-select/selected-name";
+import computed from "ember-addons/ember-computed-decorators";
+import { categoryBadgeHTML } from "discourse/helpers/category-link";
+
+export default SelectedNameComponent.extend({
+  classNames: "selected-category",
+  layoutName: "select-kit/templates/components/multi-select/selected-category",
+
+  @computed("content.originalContent")
+  badge(category) {
+    return categoryBadgeHTML(category, {allowUncategorized: true, link: false}).htmlSafe();
+  }
+});

app/assets/javascripts/select-kit/components/multi-select/selected-color.js.es6

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+import SelectedNameComponent from "select-kit/components/multi-select/selected-name";
+
+export default SelectedNameComponent.extend({
+  classNames: "selected-color",
+  layoutName: "select-kit/templates/components/multi-select/selected-color",
+
+  didRender() {
+    const name = this.get("content.name");
+    this.$(".color-preview").css("background", `#${name}`.htmlSafe());
+  }
+});

app/assets/javascripts/select-kit/components/multi-select/selected-name.js.es6

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+import computed from "ember-addons/ember-computed-decorators";
+
+export default Ember.Component.extend({
+  attributeBindings: [
+    "tabindex",
+    "content.name:data-name",
+    "content.value:data-value",
+    "guid:data-guid"
+  ],
+  classNames: ["selected-name", "choice"],
+  classNameBindings: ["isHighlighted", "isLocked"],
+  layoutName: "select-kit/templates/components/multi-select/selected-name",
+  tagName: "li",
+  tabindex: -1,
+
+  @computed("content")
+  guid(content) { return Ember.guidFor(content); },
+
+  isLocked: Ember.computed("content.locked", function() {
+    return this.getWithDefault("content.locked", false);
+  }),
+
+  click() {
+    if (this.get("isLocked") === true) { return false; }
+    this.toggleProperty("isHighlighted");
+    return false;
+  }
+});