Refactor stub_asset_proxy_setting uses in specs to be less error-prone

Right now, stub_asset_proxy_setting is used in a variety of places where they manually redo the work of AssetProxy.initialize_settings. It's difficult to do correctly, and if the logic of AssetProxy.initialize_settings were ever changed, these stubs would all need to be manually tracked down and updated — if not all caught, they probably would conceal downstream users of these settings that need to be updated too, but (due to the nature of a stub) would continue to pass CI despite being broken in practice.

Refactor the logic in the calls to this stubbing function, to either:

• Make only one site that needs to be kept in sync; or,
• Re-use the live logic so nothing needs to be synced for it to work correctly.