Bugfix: XHTML Sanitizer did not add nofollow attribute to links

While the filter actually did add the attribute, the tag attribute
filter removed it afterwards. Now it does not and we have a test.
Feature: Add 'add_target_blank' filter to Cleaner

Currently, it is not used anyhwere but can be added to cleaner settings
by plugins. Possibly a good default for the description?
Add category slug as data-attribute to category link

Additionally, this way we only have one call to cat_link and could
probably remove the macro.