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.

This is needed to make the project page work as expected. In the future, investing in ways of making those accessility features work would be good. But for now, it is broken for everyone because it both sends events to the VM _and_ scrolls the page.

Add cloud provider APIs for deleting and renaming cloud variables. Remove dataIndex from writeToServer function since it is not used.

Set VM's cloud provider to null when disconnecting from the cloud server. Add unit test for disconnecting on unmount.

This will work better if there are other query string options that need to be handled later. It also handles the distinction between ‘all’ and the other tutorials better.

Fix issue where regular cloud variable messages were throwing uncaught Json parse errors because of a newline being at the end of the message. Add test that fails without the bugfix.

Fix cloud connection logic so that cloud connections are made and cleared out appropriately. This fixes a bug where we were making two cloud connections when creating a new project and not clearing out previous connections when switching between editor and preview/player mode.

Uses randomized duration between [0, 2^k-1] where k is the connection attempt number, maxed out at 5.

I had to rework the tests quite a bit to make this easy to test, to work around the timeouts and the randomization.

The use of an "override" prop that allowed the preview modal to be hidden without actually changing the redux state was causing a problem where the block editors where not closing when modals were opening. That is because it relied on the modal reducer being accurate.

I defaulted the preview state to false to make it simpler to consume the GUI, as www will not want the preview info to be true. I made it true in the playground instead.

This fixes https://github.com/LLK/scratch-gui/issues/3710

Hopefully they are actually clarifying