Venue set assignments: validate that submissions are visible to the matching group

[melisabok]

In order to avoid releasing the assignments to the reviewers without releasing the submissions to them. We propose to validate the submissions are visible to the group of reviewers or submission reviewers, if that is not the case then an error is thrown during the deployment of assignments.

For manual assignments, I suggest to validate this in the pre process function of the assignment invitation.

Thoughts @celestemartinez ?