Mobile style for public links: files, folders, picture sets

Use in combination with the mobile-style branch in the apps repo, see https://github.com/owncloud/apps/pull/1607

 issues left:

* [x] Pictures detail view does not show action icons on mobile browsers, see https://github.com/owncloud/apps/issues/1610
* [x] using multiselect does not bring up the file actions, i.e. the »Download« action to download everything
* [x] Single image public share is scrolled to bottom because of focus on direct link input, see https://github.com/owncloud/core/issues/6817