To each link on the site...
That way it would open links in a new tab
You can do it with your browser settings.
Like in Tor browser.
Settings > General > Tab > Choose "Open links in tabs instead of new windows"
I think this option is only applicable to links that already have the "target="_blank"" attribute set, which opens links in a new tab instead of a new window. However, the suggestion made by the OP is to have the target="_blank" attribute set for all links from the forum's side.
@theymos
That was years ago and nowadays any decent browser support <rel=noopener> for <target=_blank> links.
I agree. As far as I know, the security concern that theymos mentioned in the post is no longer relevant.