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.
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