This includes a small bug where the target pane wouldn't automatically update if changes to the sprite occured while in fullscreen/player mode.

Closes #4221

Use the same implementation as onSetProjectThumbnailer which does the same thing for getting a thumbnail externally.

Since this is in a promise chain, it doesn't matter if onUpdateProjectData returns a promise or a primitive, it'll work either way

This is because when switching from project to editor view, this component gets unmounted and then remounted, but the componentDidMount seems to happen before the componentWillUnmount. Because it is dealing with a global variable, it is unsafe to always unset the beforeunload when unmounting.

Resolves #3950.

Unfortunately the design of scratch-storage doesn't make having multiple distinct POST urls easy at the moment. So for the time being, just make this request ourselves.

use blue creating screen for new projects; revised alert text for copy, remix; saving messages don't dismiss alerts

simplified autosave code per rschamp's comments; made isShowingSaveable and autosaveIntervalSecs props

I do not like how the alert component is hyper specialized for two distinct use cases. It would be much better to refactor the alerts so that the alerter can just fill the alert with whatever they want. But I do not feel comfortable making that refactor because I know ben and evelyn have gone back and forth about these alerts.

Listen to the new project changed event from the VM and keep track of the state in Redux.  Update it as appropriate.

Known issue is that the project is marked as "changed" immediately after it's loaded. This is due to asynchronous event emitting while the project data is loaded, and needs an update to the VM to fix.

Instead of saving .sb3 files to the projects server, save just the project JSON. Save assets to the assets server. This happens via the asset types being routed to different web sources by storage.