Browsers other than IE will be allowed to load the GUI.
However, if there is an error, the error boundary will display an unsupported browser message recommending they update to the latest version of a supported browser.

For the purposes of this PR, ‘supported’ means you can load the gui (anything other than IE), and recommended is just the browsers that meet our minimum requirements for Chrome, Firefox, Edge and Safari.

The playground now displays this on startup when in Scratch Desktop
simulation mode.

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.

This prevents unnecassary changes that do not matter to the player view because they were made by the project in progress (i.e. made by running the blocks, or by cloud var updates)

They were not being marked as clean when loaded from the server, causing remixes that included only minimal (or no) changes to take a very long time as it waited for hundreds of network requests to resolve. Mark them as clean when loaded from the server so we know they do not need to be resaved

This was intended as a safety check but it causes problems with some
tests.

This should have happened a long time ago, but the Fastly config prevented it

randomize sprite positions when added from image file or from library (through 'choose from library' and 'surprise')