Add a (hidden)-like indicator to assignments when "Visible during scheduled time" is used

[sophia-huynh]

When the "Visible during scheduled time" option is selected, the (hidden) label on the main Assignment page disappears for the assignment.
This is a little misleading/confusing for assignments, especially ones with only a single date for release. (scheduled) maybe be a more clearer label, or something else, rather than no label.