Add loading indicator

Whilst load times are generally quite quick, sometimes they are a bit longer.
For those times it would be nice if the page displayed a loading wheel (or similar indicator) so that the user doesn't think the page has frozen.

Relevent comment from old repository:
> "Not sure I agree, in my opinion it's better to have the page "freeze" like normal web pages instead of having a spinner momentarily appear, causing the page to blink, especially since page navigation is rather fast. We would move to use React suspense, and show a spinner if the loading takes more than 300ms or something."

Having it only appear if loading takes a bit of time sounds like a good idea IMO! :)