Move NM connectivity check file (check_network_status.txt) to a subdomain

NetworkManager in arch by default uses http://www.archlinux.org/check_network_status.txt to check for connectivity.

Today during maintenance the maintenance role was used and http://www.archlinux.org/check_network_status.txt returned a error for some time[1][2]. I think we should move the file to a subdomain as it isn't related to the main site and we can probably avoid this issue in the future.

Naming inspiration: http://nmcheck.gnome.org/check_network_status.txt http://detectportal.firefox.com/success.txt

[1] https://www.reddit.com/r/archlinux/comments/keai0g/does_anyone_know_if_this_is_normal/
[2] https://www.reddit.com/r/gnome/comments/ke9ytm/network_manager_popup/