Theme Field validation

At the launch of CMS Hub themes, the fields.json file was not validated for errors. We provided a temporary API to give developers who've already implemented themes a path to fixing issues with their fields.json file before these errors were shown in the design manager.

These errors as of August 17th are displaying in the Design Manager and the local development tools. This API no longer serves a purpose as a result and we have removed the documentation for it.

Learn more on the changelog.