Add "Hide download" to the menu of link shares

Hiding the download does not depend on other settings and it does not
affect other settings either (for example, it would be possible to hide
the downloads yet make the share editable), so a simple checkbox was
added to the menu. However, note that this option is only available for
files, but not for folders.

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