feat(pm/wiki): make external links open in new tab

Coded wanted this, and I'm fine with this... internal and interwiki
links still won't do this (which is good)

packetmix/systems/wiki/wiki.nix

···
 
         $wgEnableEditRecovery = true;
 
+        $wgExternalLinkTarget = '_blank';
+
         $wgShowExceptionDetails = true;
         $wgDevelopmentWarnings = true;
       '';