add CSS to disable + dim opacity of "Enter VR" button from being clicked until ready to enter VR

[cvan]

the "Enter VR" button should be dimmed (CSS: opacity: .5), and you should not be able to click on the button (CSS: pointer-events: none; cursor: wait) until a VR display is detected and Unity has fully loaded. when all is ready, brighten the button (CSS: opacity: 1) and allow clicks (CSS: pointer-events: auto).

[cvan]

I looked into this, and this improvement will improve the experience for a very narrow subset of users (i.e., those who have a VR headset connected/mobile headset and are in 2D mode interacting with the scene, about to enter VR mode). I will still add this though. I just think the fix necessitates a larger rearchitecture of the UI (such as issue #59).