In e1d27400f5 we started running the splash-screen JS through terser, which removed the trailing newline from the `sourceMappingURL` line.
Adding a reliable end-to-end test for this isn't possible because our testing environment doesn't use terser.
This lets us use all our normal JS tooling like prettier, esline and babel on the splash screen JS. At runtime the JS file is read and inlined into the HTML. This commit also switches us to use a CSP hash rather than a nonce for the splash screen.