Refactor epub reader progress calculation

[overhacked]

The code in epub-progress.js was causing the epub file to be loaded twice over the network, because it created a second instance of the epub.js library. I moved all the progress percentage display code into epub.js. Only the locationchange polyfill code was left in the separate file, renamed to locationchange-polyfill.js.

I rewrote the percentage progress code to use epub.js events, giving more succinct and readable code. Also, I added localStorage caching of the epub location calculations, required for displaying percentage progress, which can take up to 60-90 seconds to calculate on longer ebooks. This localStorage caching approach is recommended by epub.js in the locations.html example in their repo.

[overhacked]

I think that this polyfill might be unnecessary now. The previous percentage location update code used...

calibre-web/cps/static/js/reading/epub-progress.js

Lines 41 to 44 in efc7ce3

	window.addEventListener('locationchange',()=>{
	let newPos=calculateProgress();
	progressDiv.textContent=newPos+"%";
	});

...to update progress, but this new code uses epub.js events:

reader.rendition.on('relocated', (location)=>{
    let percentage = Math.round(location.end.percentage*100);
    progressDiv.textContent=percentage+"%";
});

So, maybe locationchange-polyfill.js can be removed altogether?