PERF: Improve workbox loading strategy (#22019)

Previously workbox JS was vendored into our git repository, and would be loaded from the `public/javascripts` directory with a 1 day cache lifetime. The main aim of this commit is to add 'cachebuster' to the workbox URL so that the cache lifetime can be increased.

- Remove vendored copies of workbox.
- Use ember-cli/broccoli to collect workbox files from node_modules into assets/workbox-{digest}
- Add assets to sprockets manifest so that they're collected from the ember-cli output directory (and uploaded to s3 when configured)

Some of the sprockets-related changes in this commit are not ideal, but we hope to remove sprockets in the not-too-distant future.

lib/ember_cli.rb

@@ -16,6 +16,13 @@ module EmberCli
         assets +=
           Dir.glob("app/assets/javascripts/discourse/scripts/*.js").map { |f| File.basename(f) }
 
+        if workbox_dir_name
+          assets +=
+            Dir
+              .glob("app/assets/javascripts/discourse/dist/assets/#{workbox_dir_name}/*")
+              .map { |f| "#{workbox_dir_name}/#{File.basename(f)}" }
+        end
+
         Discourse
           .find_plugin_js_assets(include_disabled: true)
           .each do |file|
@@ -62,4 +69,13 @@ module EmberCli
         JSON.parse(ember_source_package_raw)["version"]
       end
   end
+
+  def self.workbox_dir_name
+    return @workbox_base_dir if defined?(@workbox_base_dir)
+
+    @workbox_base_dir =
+      if (full_path = Dir.glob("app/assets/javascripts/discourse/dist/assets/workbox-*")[0])
+        File.basename(full_path)
+      end
+  end
 end