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

Fix public download link when zip download is disabled

Fixes #8590 
    
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.

Please review @MorrisJobke @icewind1991 

@Tie-fighter @ghiamar @enoch85 can you help testing this ?