WIP: Feedback attachment

https://github.com/geoadmin/mf-geoadmin3/issues/1522

[Test Link] (http://mf-geoadmin3.dev.bgdi.ch/gal_feedback_attachement/src/?X=202235.27&Y=752102.91&zoom=1&lang=en&topic=ech&bgLayer=ch.swisstopo.pixelkarte-farbe)
yes, I think it is more secure, we make sure it's a POST request here:
https://github.com/geoadmin/mf-chsdi3/blob/gal_feedback_attachement/chsdi/views/feedback.py#L12