Allow markdown rendering in transaction comments. (#621)

This commit allows transaction comments to display newlines and URLs in
a useful way, helping when additional data (such as a reference to a
ticket system or a longer discussion) is required.
This PR also prevents pretix from having to bring its own chat system ;)

src/pretix/plugins/banktransfer/views.py

@@ -106,10 +106,13 @@ class ActionView(View):
             return self._retry(trans)
 
     def _comment(self, trans, comment):
+        from pretix.base.templatetags.rich_text import rich_text
         trans.comment = comment
         trans.save()
         return JsonResponse({
-            'status': 'ok'
+            'status': 'ok',
+            'comment': rich_text(comment),
+            'plain': comment,
         })
 
     def post(self, request, *args, **kwargs):