[w3c/manifest] Codify that the related_applications member should be consulted when installing a web application (#707)

Currently, the [related_applications](https://www.w3.org/TR/appmanifest/#related_applications-member) member is very generally specified. Perhaps we should tighten the requirements on UAs here and enforce that UAs SHOULD only offer a web application for installation if the related_applications member is empty OR if none of the listed and compatible `related_applications` are installed.

This is independent of [prefer_related_applications](https://www.w3.org/TR/appmanifest/#prefer_related_applications-member), which tells the UA that a web application should never be offered for installation over the top of a related application (even if the related application ISN'T installed).

A question here is whether sites have listed `related_applications` but NOT specified `prefer_related_applications` to be true. Is there any reason why sites might do this, but NOT consider their web app and related app interchangeable? One example I've found is [Google Maps](https://www.google.com/maps/preview/pwa/manifest), but there it's probably fine to say that the native app and web app are interchangeable. I'm not sure if this holds in general so I'm interested in hearing thoughts.

I'm happy to draft some spec language if people think this requirement is sound.

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