Accessibility compliance: Improve share dialog accessibility

[srietkerk]

I've made a couple of changes with this PR, but I've done so much fiddling and changing and testing across these commits that it's hard all of what's changed. The code in this PR does fix the following issues:

Fixes https://github.com/microsoft/pxt-microbit/issues/5468 Fixes https://github.com/microsoft/pxt-microbit/issues/5472 Fixes https://github.com/microsoft/pxt-microbit/issues/5488

I also did some work for this issue https://github.com/microsoft/pxt-microbit/issues/5471. I finagled with this a bit and got it to a place that looked okay, but it really made the other zoom and screen experiences just bad, so I scratched the progress made. I did make a change, though, that if a screen is small enough, I change the height of the modal overlay to be 95% of the screen so more of the modal is usable.

It will be easier to see these changes just by playing with difference zooms, screen sizes, and resolutions. Let me know if there are other things that I should adjust. Upload target: https://makecode.microbit.org/app/44c2bdbef7436f5a3215b42b34cf672cb13298ac-2ae3b742d0#editor