allow site administrators to modify PUBLIC_EXTENSIONS_REPOSITORY via configuration

CiviCRM establishes a network connection to the value of PUBLIC_EXTENSIONS_REPOSITORY to download information about available extensions. This is set in the CRM_Core_Extensions class as a constant. It's also set to an URL that does not use https.

If this variable was a site wide constant that could be set in civicrm.settings.php, then site administrators would be able to override this value. Ideally it could be set-able to NULL to prevent civicrm from including remote repositories.

If this seems like a good idea, please feel free to re-assign to me.