Implement a notice to Windows XP users for end-of-life.
Closes #666
Update XP warning copy.
Implement #666 using better logic and only showing the XP warning at UNL

The XP warning uses a service to check on campus IPs. This service is
currently only available via the insecure protocol.
Merge pull request #668 from kabel/fr-666

Implement a new warning for Windows XP