Add view to confirm tokens via Links in Emails.

This adds support to confirm requests sent to users by clicking on links which some consider to be more user friendly than replying to emails with confirmation tokens.

This URL (/list//confirm/?token=) would then need to be added to the templates using $token as a variable, to be filled in by Mailman Core when sending out email.

Fixes #459 (closed), #141 (closed)