WIP: Feedback attachment

https://github.com/geoadmin/mf-geoadmin3/issues/1522

Current implementation accepts only one attachment.
Similarly to the Import KML, this feature is not available in IE9.

[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)


I wasn't sure about that for POST requests using `serviceUrl` in
https://github.com/geoadmin/mf-geoadmin3/blob/gal_feedback_attachement/src/js/FeedbackController.js#L9
should do the trick then?