I've added a new feature that allows each user to select whether they want this on or off by default.
For any external link you can always right-click and choose open in new tab/window but this new option
allows you to also have the left-click behaviour do whichever you prefer.
Just go to your User Control Panel - Board preferences - Edit global settings and
you should see the new options at the bottom of the page.
The User Control Panel is accessed by clicking on your user ID in the top right corner.