Upgrade React to v18

[Mr0grog]

There were a few minor code changes needed for React 18 to work, and a lot of changes needed to keep tests working (most really have no major runtime impact, although a handful do probably make things slightly more robust).

A lot more stuff under the hood in React has become async, so a lot of these test changes are about timing and making sure a component fully renders and goes through its loading lifecycle (and cancels async API calls if the component is prematurely unloaded, as they are in some tests). Similarly, lots of test code also needed to now be wrapped in act(() => (do a thing)).

I also updated some of the JS class syntax for a couple components that needed more invasive work for data/state loading while I was in there (for example, diff-view.jsx).

[Mr0grog]

I think this is probably good to go. I’d wanted to look into using the new <Suspense> component, but the docs (even for React 19) still say that “Suspense-enabled data fetching without the use of an opinionated framework is not yet supported” and do not document publicly how to manage Suspense’s state except via the use hook, and we don’t generally utilize hooks because this app largely predates function components. So no suspense for now. 🙁