Remove target="_blank" links or sanitise them

[danielcompton]

There is a vulnerability with opening target="_blank" links without adding rel="noopener noreferrer". I'd argue we probably don't want them at all, but if we do then we should also add the rel stuff too.

https://medium.com/@jitbit/target-blank-the-most-underestimated-vulnerability-ever-96e328301f4c#.5dcfxcy6q
https://news.ycombinator.com/item?id=11631292