FEATURE: Normalize the service worker route (#8359)

* FEATURE: Normalize the service worker route

Update cache headers so they are not immutable outside of the rails app

Add the ability to purge the service worker cache from localhost

Rails -> nginx will pass immutable flags so the file is cached until reloaded.
In most cases, nginx will have its cache flushed on rebuild (new image)

For those needing dynamic re-caching (such as upgrading via the UI),
a rake task for flushing the service worker script is provided
through `assets:flush_sw`

app/helpers/application_helper.rb

@@ -456,8 +456,6 @@ module ApplicationHelper
   end
 
   def client_side_setup_data
-    service_worker_url = Rails.env.development? ? 'service-worker.js' : Rails.application.assets_manifest.assets['service-worker.js']
-
     setup_data = {
       cdn: Rails.configuration.action_controller.asset_host,
       base_url: Discourse.base_url,
@@ -465,7 +463,7 @@ module ApplicationHelper
       environment: Rails.env,
       letter_avatar_version: LetterAvatar.version,
       markdown_it_url: script_asset_path('markdown-it-bundle'),
-      service_worker_url: service_worker_url,
+      service_worker_url: 'service-worker.js',
       default_locale: SiteSetting.default_locale,
       asset_version: Discourse.assets_digest,
       disable_custom_css: loading_admin?,