Minimize the amount of code loaded on each page even further

- Move the logic for basic mode ("Origin Pull mode" in the UI) into cdn.basic.inc
- Move the logic for advanced mode ("File Conveyor mode" in the UI) into cdn.advanced.inc
- Use require() instead of require_once() to load include files, as per #320493: Use require() not require_once() in the registry
- Create a cdn_include() function that uses require() and makes it easier (and more elegant) to load CDN module's include files