Edit: forgot my manners welcome to the forums, hopefully Nodebb will be everything you need from a forum platform.
Hotly debated subject and something I have to disagree with. If a user is clicking a link, they want it to open in that window. That is the default behaviour on all browsers on all good websites, the target attribute isn't XHTML 1.1 valid either so it's hacky. Forcing all links to open in a new tab is hugely annoying, as clicking the back button would do nothing, this is bad UX and should be discouraged unless absolutely necessary. (When filling out a form for example, and even then, tooltips are a better option).
If someone wishes to open links in a new tab, then they can right click that link and tell it to open in a new tab, or a new window. Alternatively Shift+click will achieve the same thing.
However, in your profile settings is an option to open outgoing links in a new tab. It is off by default for each user and is set by each user in their settings, if they want it, it's there. pro tip: you can edit other people's settings to turn this on for them if they have specifically requested that feature.
I could go on and on about why it's discouraged, but it all boils down to this point here
I don't want to pull traffic away from my forum.
If your forum is worth reading, they'll click the back button, forcing members to stick around via bad UX will result in frustration and a high desire to close the website altogether.