Add generation of a publicly accessible download directory with a non-guessable name

[chill1n]

For public sharing of photos, videos (or documents), please provide for admin users the following:

1. generation of non-guessable directories (e.g. SHA-256 hash) with a certain validity interval (begin timestamp + end timestamp). if no timestamp is specified, no timestamp applies
2. set read or write permissions for such a directory (per user/group)
3. allow access to this type of directory without a separate login