Add BooleanField that toggles public display of MemberGroup members

One-sentence description

Add BooleanField that toggles public display of MemberGroup members

Why?

Because some nicknames are weird. And we've got sponsors and other externals to care about. Plus: external people don't need to know names and they can't even see profiles.

Current implementation

Show members on board/committee/society pages

Desired implementation

Do not show members on board/committee/society pages when you're not authenticated and the added field is checked/non-checked. Better to explicitly check the field for public display probably.