Remove noopener noreferrer

[michamos]

Paul Ginsparg from arXiv reported that they didn't see any traffic from INSPIRE recently. The culprit is the noreferrer attribute we've added in #375 to external links following a vulnerability report. According to https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel/noopener, this turns out not to be needed any longer on modern browsers, so we should revert this and drop the noopener noreferrer attributes from all links.