Add proof of concept cloud-compiled share page

[mmoskal]

Requires microsoft/pxt#5858
Go to say http://localhost:3232/70131-58277-87184-36217 to test it (after rebuilding pxt, and re-rerunning pxt serve --rebundle)

• while the splash page is up, we are already loading simulator and compiled code in background
• the reset button goes back to the splash page

TODO:

• styling for splash page
• add Play button over script thumbnail
• abuse report UI
• do we want to use semantic UI in here?
• prevent control.reset() from going to the splash page
• the cloud script APIs should indicate if there is a thumb nail and we should use that when rendering this page

[mmoskal]

@riknoll you may want to use this for your control update (or maybe not, not sure). It has less layers than the usual share page.

[pelikhan]

Semantic UI is bloated - let’s keep this page lean

[mmoskal]

test page: https://arcade.staging.pxt.io/37372-65194-89912-62054

@shakao maybe you want to take a stab at styling this? Right now it looks seriously ugly, but is very responsive. It's easy to test locally - see top comment