Make getLayout cacheable

Problem/Motivation

At present LayoutController::getLayout returns JsonResponse and not CacheableJsonResponse.
We're already collecting cache metadata. We should use it and make the page cacheable.

Steps to reproduce

Proposed resolution

Work out how to get cacheability metadata from the tempstore/section storage.
Add that to the existing cacheable metadata from normalizer.
Switch from JsonResponse to CacheableJsonResponse

Remaining tasks

User interface changes

API changes

Data model changes