Instructor Edit Session Page: If make session visible is later than submission opening time, automatically default to opening time

[weiquu]

After creating a session, the visibility is before the submission opening time:

When editing the session, it's possible to have the submission opening time be set earlier, in which case the session visibility time becomes invalid:

I think that in such a scenario, having the session visibility time default to the submission opening time is appropriate behaviour. This is somewhat similar to the case when submission opening time is set to be after submission closing time - in that scenario, submission closing time defaults to submission opening time.

Also open to discussion as to whether this behaviour is expected by the user or not