initial backport of #6777 - mobile public pages - to stable5
add missing mobile.css
fix undefined index
unset box-shadow for mobile devices, unsupported in all but the latest mobile browsers
fix scrolling for mobile browsers
Merge pull request #7185 from owncloud/backport_6777_mobile_style_for_public_pages_stable5

initial backport of #6777 - mobile public pages - to stable5