Allow message to be prefilled on feedback page

So that people don’t have to remember what they are supposed to be
asking for and go to the effort of typing out the message.

app/main/views/feedback.py

@@ -134,6 +134,9 @@ def feedback(ticket_type):
             abort(500, "Feedback submission failed")
         return redirect(url_for('.thanks', urgent=urgent, anonymous=anonymous))
 
+    if not form.feedback.data:
+        form.feedback.data = request.args.get('body', '')
+
     return render_template(
         'views/support/{}.html'.format(ticket_type),
         form=form,