Use a dedicated thread to run Scheduler::Defer
This avoids blocking of a worker during operations that require waiting.
In particular uploads risked blocking a unicorn.
This also add a queue "length" that discourse prometheus consumes.
* 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()
```
* Add user_home configuration option
* Use the new user_home preference to actually show the right home page
* Fix trailing whitespace
* Update user_option_serializer.rb
* Fix JavaScript default homepage tests
* Use an object instead of a giant switch
* Remove trailing whitespace
* Make the default `user_home` set to `null` instead of `0`
* Rename user_home to homepage_id
This makes Discourse compliant with latest Google PWA requirements,
so we get the App Install banner back.
Should bump our Lighthouse PWA Audit score to 11/11.
* set up static offline.html route and service worker for Android Web App Banner
* add viewport meta tag to offline view for android app banner
* add i18n support for offline.html pages, cleanup
* fix html syntax, add page title, remove license for service-worker.js
