Fix cache control header, files are public, /content/images/* results in 404

The current Cache Control header uses 'maxage' instead of 'max-age' which means that browsers don't honor it. This fixes that, and sets the timeout to be one year from now, which should be fine because every new file uploaded is a unique URL.

Maybe I didn't set my bucket up properly, but I needed to make every file public manually until I added the 'public-read' ACL. I imagine others will run into this.

Lastly, the noop provided by `serve` actually results in a hang whenever a user requests something under the /content/images/ base path. This change makes those requests result in a 404 instead.