Prevent Z-Link indexing by search engines

[jxjj]

Possible solution: set X-Robots-Tag header.

See:

• https://tdx.umn.edu/TDNext/Apps/29/Tickets/TicketDet.aspx?TicketID=1051628
• https://thoughtbot.com/blog/block-web-crawlers-with-rails

[jxjj]

Discussed, and we're not sure what the right behavior is yet. Moving to discussion for now.