Enable brand views to respond to model changes from OPE

Description

As a partner developer building a rich client using JavaScript, I want to control the DOM with my chosen JavaScript framework. So the editor/marketer can use OPE to edit content processed by my JavaScript without breaking my view.

New property value: data-epi-property-render="none"
New event to listen to: subscribe("beta/contentSaved", callback)
See blog post: https://world.episerver.com/blogs/john-philip-johansson/dates/2017/10/taking-control-of-client-side-rendering-in-ope-beta/