Minor fixes as a follow-up to #1340
* Assembla is HTTPS-only
* Mailing list is members-only, so link to the group instead and call it
"mailing list" instead of "email"
* getontracks.org's canonical URL has no www
* Make the same changes to the mobile footer
* The forum is no longer referenced, so purge from translations
The link to the forum is gone. The email link points to the new google
groups mailing list. The contribute link points to
http://getontracks.org/developmentFixes#1340