Disable add-Button in studydocs

As discussed with @lutzsa: Since the autocomplete from the website doesn't work in the admintool. The add-button in the studydocs subpage might be disabled as well.

I think the way to go here is having the same kind of upload options like on the website, or have a redirect to the website.

added low_priority label

Why don't we redirect the button to the website until we have the same options. Just that there aren't generated too many fields with typos ;)