* Move from env variables to SiteConfig
Related to #5384
This PR only deals with the first remaining part outlined in the issue, starting to use existing SiteConfig keys instead of the env variables.
* Restore Envfile to original version for now
* Initial work for streaming app shell way of using serviceworkers
* Close in on serviceworker adjustment finalization
* Add docs and loading indicator
* Remove useless code
* Don't run on API
* Update app/javascript/contentDisplayPolicy/hideBlockedContent.js
* Fix small details
* Don't run serviceworker.js code in test env
* Fix JS in serviceworker
* Move private keyword to proper place in async controller
* Change shell_version to HEROKU_SLUG_COMMIT
* Update caching config
* Add test for async_info/shell_version