[CORE] Move public resources to a /public directory

[CORE] Move public resources to a /public directory
Advantages:
* Increases security by preventing direct access to file/
* We are careful and have a defined('GNUSOCIAL') || die() to prevent
  direct access to GS files, but we may miss one or a vendor/extlib may
  not be as careful
* Improves directory structure - It's more natural to physically
  separate what is public from what are GNU social resources