Don't set canEdit to true if the answer is a legacy answer

Closes #168 (closed)

I doubt that it was intended this way and the bug was more likely that a new legacy answer was created, but after discussing with @zibungj a bit I think that this is the most sensible way to do it. If the user intended to edit the original answer he can still delete it and copy the content.

The other possible way would be to modify def set_answer(request, oid): so that it modifies the unique legacy answer per answer section