Put editors picks for collection

This adds `editorsPicksForCollection` to`ConfigAgentTrait` to allow you to figure out what paths consider this `collectionId` as `editorsPicks`.

The selection criteria is just the top collection for a given path.
The key `editorsPicks` pushed to the `Content API` is a list of paths.

This cannot be merged until the `ContentAPI` team have merged their side of the work, which should be today.

@stephanfowler @robertberry @jennysivapalan