UserTrophyAdd: Prefill description with the selected trophy's description when 'useCustomDescription' is checked

In order to ease creating trophies with a consistent custom description the description field could be prefilled with the description of the selected trophy when useCustomDescription is checked.

As an example this could be useful for a monthly contest where the description includes the topic and month of the contest and is otherwise phrased identical: „{$username} won the <contest name contest> held in <month>, <year>“.