URL cron event improvements

[johnbillion]

• Introduces support for a CRONTROL_DISALLOW_URL_EVENTS constant which behaves the same as CRONTROL_DISALLOW_PHP_EVENTS does for PHP events:

  If the CRONTROL_DISALLOW_URL_EVENTS constant is defined and set to true then URL cron events will be disabled completely. Any existing URL cron events will remain in place but their URL will not be fetched, and no URL cron events can be added, edited, or manually run. Users with permission to edit URL cron events will still be able to delete these events.

• Better handling of URL cron events in CSV exports