referer: do not send referrer when opening a link
referer: do not send referrer when opening a link

This change augments what was already done in 54e0f2de5b20 (PR with discussion
at #3636).

For documentation about the meaning of "noopener, noreferrer", see:
https://developer.mozilla.org/en-US/docs/Web/API/Window/open#Window_functionality_features

aptivate/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite