Add note that uploaded data is private

[normanrz]

Detailed Description

I had conversations with a number of users that were assuming that if you upload data to Webknossos, it is public immediately. That is, of course, not true so we should educate users about that. Maybe we should put a note in the upload screen? Maybe also somewhere on the website. I think the docs are already clear about this.

[knollengewaechs]

I will add a note to the upload screen. If I understood correctly, Albane (did not find her github username :( ) is taking care of the website!