As a SPA developer I want to run the page editor on a SPA on an external server, so that developing with it is easier, maintaining it is easier, and so that authors get a perfect preview

Notes:

Currently, it is not possible for the page editor to directly operate on a page loaded from a different domain in an Iframe due to browser security restrictions. 

In a truly headless scenario, the SPA may be running on a external server. For example React apps often run on a Next server, Vue apps often run on a Nuxt server.

Even if not using any of these frontend servers, developers benefit when they dont need to always have this extra step of deploying the SPA to the magnolia light module when there are changes. 

Acceptance criteria:

• Developer only needs to provide the URL of the external server in their template definition.
• Works with "nextjs".
• Works with "gatsby".

There is an Incubator module which does this:

https://wiki.magnolia-cms.com/display/SERVICES/SPA+renderer+extended

Notes from meeting which describe rationale of "spa-renderer-extended":

https://wiki.magnolia-cms.com/display/TH2/2020-10-07+External+SPA+Braindump