This breaking change was originally a deprecation fix for overriding a computed property `none`.
There are 4 uses of `rootNone` in core and "all-the-plugins":
1. in discourse-chat-integration, admin-plugins-chat-edit-rule.hbs - changed behavior, that I'd consider a fix - `rootNoneLabel` is now used regardless of `siteSettings.allow_uncategorized_topics` value, which I believe was an originally intended behavior (i.e. it most likely hasn't been tested with disabled uncategorized topics)
2. in discourse-slack-official, plugins-slack.hbs - the same as 1.
3. in core, edit-category-general.hbs (in this PR) - no change in behavior
4. in discourse-googlebooks, edit-category-general.hbs - no change in behavior (since `allowUncategorized="true"` is also passed as an argument)
* 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()
```
