Honour "hide download" in the public share page

When the "hide download" property of a share is set the public share
page will not show the download button nor the menu with the download,
direct link and "Add to your Nextcloud" actions; the "downloadURL"
hidden field will not be included either in the generated HTML.

Despite that, note that the "downloadURL" parameter is still set and
passed to the template, as this could be needed anyway to generate
previews (for example, of audio files).

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>