Don't force external links to open in a new tab

Overriding user preference is a bad idea from a usability point of view. Just allow people to decide for themselves whether they want to navigate away to another page or open the link in a new tab using the controls provided them by the browser (eg. cmd-click, or the options in the right click menu).

This also fixes the situation where someone might want to open an external link in the same tab, which is currently made impossible.
Merge pull request #3288 from Primigenus/patch-3

Don't force external links to open in a new tab