Add support for the cdn reporting parameter

[danielrozenberg]

This changes the reporting bucket names to include the CDN, and be more explicit by renaming buckets that used to be called "CDN" to be called "Google Cache", as this is what they were actually reporting.

The changes are summarized here:

	Served from the Google Cache	Served from publisher origin, JS from cdn.ampproject.org	Served from publisher origin, JS from ampjs.org
Before this PR	CDN ${channel}	Origin ${channel}	Origin ${channel}
After this PR	Google Cache > ${channel}	Publisher Origin (https://ampproject.org) > ${channel}	Publisher Origin (https://ampjs.org) > ${channel}

Minor tag-along changes:

• Replaced a few export default situations with named exports
• [a, ...b] instead of [a].concat(b)