External links should opened in new tab/window

[Gandalf-the-Grey]

Currently external links are opened in same tab/window which drives users out of the site instead of keeping them here (so for example, users will go to enjoy external site, but will not have a chance to upvote post that directed them there)

Related to: #710

but when using target='_blank' security issue can be avoided by proper use of rel="noopener noreferrer"

[roadscape]

FTR, this was not done earlier because noopener was found unreliable in some cases. https://mathiasbynens.github.io/rel-noopener/ seems to have been updated to reflect the latest state of noopener, and still lists some caveats;

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