CiviMail: Ampersands in tracked URLs are no longer decoded (regression)

Amersands in trackable URLs get encoded before they are saved.

The extern\url.php script used to call CRM_Utils_System::redirect, which rewrites '

&

to

&

However since CRM-17953 it doesn't any more, so any URL with an ampersand is now broken =[

Proposed fix:
Why can't we use CRM_Utils_System::redirect, we call Utils_System again below it, so it must be available to Civi - if not then let's include that file so we can use it.