Fix public download link when zip download is disabled

When zip download is disabled, the public download action defined in
public.js is overridden by the one in fileactions.js because of the JS
loading order.

This quick fix prevents the override to happen when a download action is
already defined.

There are additional changes that were required to make the download
action icon work when registered from the public page.
Merge pull request #9015 from owncloud/stable6-downloadwithdisabledzip

[stable6] Fix public download link when zip download is disabled