Support OPE in decoupled sites cross domain

Description

Up until now, on-page editing required sharing the domain between the delivery site and the management site; developers had to ensure that the two sites set the same document.domain.

The new On-Page Edit feature does not require access to the document in the delivery site because the communicationInjector.js script (included in the delivery site) inspects the rendered page and uses postMessage to report back rendered properties and their positions to the management site. This lets the management site show overlays and add editors to the properties.

This release primarily supports server-side-rendered content and the next version will improve upon the support for Single Page Applications where content is rendered on the client.