We weren't using this very much and introduces a dependency between
discourse-common and discourse which makes moving to yarn workspaces
more difficult.
In the future we might user ember-addons properly but for now it's
easier to move the code into discourse-common.
Note the old folder is still there because at least one plugin was still
requiring the old files. It will be removed in the future.
* 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()
```
