Replace navigator.xr.supportsSession with navigator.xr.isSessionSupported to follow the latest WebXR API spec

This reverts commit 46e7b4a2.

WebXR's requestReferenceSpace only works if the corresponding feature
was requested at session creation time. For simplicity, just ask for
the interesting ones as optional features, but be aware that the
requestReferenceSpace call will fail if it turns out to be unavailable.
('local' is always available for immersive sessions and doesn't need to
be requested separately.)

In the vr samples, handle rejection of getVRDisplays for the case when the API is available but WebVR is not supported

Propagate VR-mode upon link traversal so the the user doesn't have
to manually click on the 'enter VR' button each time.