Cache assets and API calls using a service worker

[mshenfield]

We already have a service worker - it's just not doing anything. We should cache assets and API requests - this could really reduce un-necessary load to the completions endpoint.

[mshenfield]

For /api endpoints, the main goal is to reduce calls to /api/completions and api/algebra when someone is
• backspacing
• retyping the same thing
• swapping

This doesn't have to live for longer than their visit - is there a way to empty a service-worker cache when someone leaves the page? This is probably better to implement in an API client with an in-memory cache.

EDIT This portion was completed in de5262a