* 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`
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