Backport 6777 stable6

This PR backports the changes from https://github.com/owncloud/core/pull/6777 to stable6, making the public shared pages more accessible on mobile devices with smaller screens.

I had to manually go through the commits and reproduce their original intent because the original PR merged master two times, making cherry picking hard at best. I also backported some of the html cleanup but not everything. 

I tested the upload and download behavior in chromium and it works as expected. Nice work by @DeepDiver1975 @jancborchardt.

PR for apps repo is https://github.com/owncloud/apps/pull/1645