linkbacks: fix external url creation

* Fixes the order of keywords arguments in `url_for` that breaks if one
  tries to generate external link.  (closes #1707)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Sorry, it should have been `url_for('weblinkback.sendtrackback', recid=1, _external=True)`