Make extension entrypoint match repository name

This patch adds a symbolic link to 'FundraisingEmailUnsubscribe.php' which
points to 'FundraiserUnsubscribe.php'. (The Jenkins extension autoloader
expects the PHP entrypoint to have the same name as the extension.)

Change-Id: I10a7da67bb6a22edbde56fc83188dc4ffe0cd813