Overhaul VisibilityModel?

[GeoffreyY]

Currently VisibilityModel behaves as follows:

• is it's public everyone have access
• otherwise in private mode only people on whitelist have access
• timed is public after release_time, private before

I feel like this is a bit wonky, and can't handle more complex behaviours. But I'm not sure what structure should the new model have.

One possible model is that we have a RawVisibility, and a CombiningVisibility, so we can have:

Combine - OR

Combine - AND

Raw - List - TA
Raw - Time - Monday

Combine - AND

Raw - List - Students
Raw - Time - Wednesday

Raw - Time - Friday