Enhancement: Upload Request similar to Dropbox's File Request

[Forceu]

[Forceu]

Duplicate of #107, but leaving it open for now.

Unfortunately guest uploads did not make it into v2.0.0.
As a workaround, it would be possible to create a new user with the recent user management update and send the credentials. The new user can by default not view any files from other users.

[Forceu]

Added in 325849b