Display help buttons only on desktop version

This PR displays  `ga-help` directive only on desktop version. That fixes #1437  and also with a css bug on safari.


Test [here] (http://mf-geoadmin3.dev.bgdi.ch/dev_over/prod/?lang=fr&topic=ech&bgLayer=ch.swisstopo.pixelkarte-farbe&X=190000.00&Y=660000.00&zoom=1&catalogNodes=457,477&layers_opacity=0.75&layers=ch.bafu.biogeographische_regionen)