Restore wikilink2 to included sidebars

[takuy]

Is your feature request related to a problem? Please describe.
As described here: 8241775#r42806622

This impacts a local plugin/customization of mine that depended on being able to detect wikilinks that did not link to actual pages (ie had the class wikilink2) - it's a big deviation from the standard DokuWiki functionality as we ll.

Describe the solution you'd like
Perhaps this could be a config option for "don't style links to non-existent pages differently" or a config to define a different class to replace wikilink2.

Describe alternatives you've considered
I've commented out the line of code that does this currently.

Additional context
Add any other context or screenshots about the feature request here.

[giterlizzi]

Re-added in 4a8e939 in dev branch.