Make an option for to make lists "public"

[sambaxter]

We would like to have an option for people to make their lists public, which would mean anyone could view their list without having to sign in. This would be only as a "viewer", not an editor (that would still require permissions and being logged in).

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• [x] Modify data model to let lists be public

  156

• [ ] Modify data model to allow for shared annotations

  158

[nawatts]

We probably want to require these be approved by an admin before adding them to a page of public lists.