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/tasks/javascript.rake

@@ -87,32 +87,6 @@ def dependencies
       source: "@discourse/moment-timezone-names-translations/locales/.",
       destination: "moment-timezone-names-locale",
     },
-    { source: "workbox-sw/build/.", destination: "workbox", public: true, skip_versioning: true },
-    {
-      source: "workbox-routing/build/.",
-      destination: "workbox",
-      public: true,
-      skip_versioning: true,
-    },
-    { source: "workbox-core/build/.", destination: "workbox", public: true, skip_versioning: true },
-    {
-      source: "workbox-strategies/build/.",
-      destination: "workbox",
-      public: true,
-      skip_versioning: true,
-    },
-    {
-      source: "workbox-expiration/build/.",
-      destination: "workbox",
-      public: true,
-      skip_versioning: true,
-    },
-    {
-      source: "workbox-cacheable-response/build/.",
-      destination: "workbox",
-      skip_versioning: true,
-      public: true,
-    },
     {
       source: "squoosh/codecs/mozjpeg/enc/mozjpeg_enc.js",
       destination: "squoosh",