Make layout strategy arguments consistent

[gadomski]

As described in #919 (comment), there's some alignment needed in how layout strategies are passed in to methods. Specifically:

• Catalog.add_item has None for a default layout strategy, but functionally it defaults to BestPracticesLayoutStrategy
• Catalog.add_item's strategy argument is undocumented
• Catalog.add_items could/should take a similar strategy argument
• Same for Collection methods of the same name