Add setStackable to ObjectItemList and use in Config

Summary:
This adds a new method for rendering the object list as a stackable set of items. Good for certain renderings like Config.

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Test Plan: Review list on iOS, Chrome, FF.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4637