Attempt to serve avatars from cache then update cache

_serverAvatars.js

...

function serveAvatar(request) {
  // Avatar urls look like:
  // avatars/sam-2x.jpg
  // But storageUrl has the -2x.jpg bit missing.
  // Use this url to store & match the image in the cache.
  // This means you only store one copy of each avatar.
  var storageUrl = request.url.replace(/-\dx\.jpg$/, '');

  // TODO: return images from the "wittr-content-imgs" cache
  // if they're in there. But afterwards, go to the network
  // to update the entry in the cache.
  return caches.open(contentImgsCache).then(cache => {
    return cache.match(storageUrl).then(response => {
        return response;
    }).then( response => {
        if (!response) {
            return fetch(request).then(networkResponse => {
                cache.put(storageUrl, networkResponse.copy());
                return networkResponse;
            });
        }
        fetch(request).then(networkResponse => {
            cache.put(storageUrl, networkResponse);
        });
    });
  });
  // Note that this is slightly different to servePhoto!
}
...