Shorten (or alias) long (un-,re-)subscribe URLs so that they're shorten than 65 characters

Currently, the (un-,re-)subscribe URLs generated by CiviMail can be longer than 65 characters; some brain-dead email clients tend to break such URLs by wrapping them.

We should either make sure we generate shorter URLs (this might not be trivial, as the 'static' part of the URL can itself be quite long) or create an aliasing service that would have short URLs itself, and would redirect to the user to the proper, long URL when visited (much like tinyurl.com). If we go with the latter, we can consider using the extern/url.php code we already have.

See the original threads:
http://forum.civicrm.org/?topic=3130
http://forum.civicrm.org/?topic=3133