Limit assignment visibility options for assignments with group submission

Improve the user interaction when setting up a group submission:

• When "Are submissions for a group?" is checked, the option of "display to site" is grayed out, and "Display to selected groups" is automatically selected.
• Then, if "Are submissions for a group" is unselected, "Display to selected groups" remains selected (because assignments requiring individual submission can still be made visible to only certain groups), but "display to site" is then ungrayed.