Move printable layout to static handling; move HTML out of template

[lizadaly]

This makes a number of simplifying and performance-enhancing changes to reading mode. It also firmly breaks reading mode away from PagedJS/PDF rendering in favor of choices that benefit screen reading only.

This does not truly break PDF output—you can still get a file from the export button—but that file looks very primitive now. I'm going to add back in PDF-centric styles in further work.

This PR:

• Puts the book content directly into the browser-viewable DOM, rather than in a template element. The former step was an optimization designed to inject annotations synchronously before PDF rendering; now the content goes straight to the browser and the annotations are added more opportunistically.
• Remove the bundled common resources from reader view; this was a big payload that was unused.
• Makes internal IDs from case content unique across the document so internal hyperlinks continue to work.

And lots of small-ish performance optimizations:

• Font preloading
• Add annotations using insertAdjacentHTML rather than insertAdjacentElement (Mozilla says it's faster and my benchmarks agreed)
• Uses requestIdleCallback for a few mutations that don't visibly affect layout

Lighthouse metrics before/after

Before

After

(The absolute numbers vary a lot between casebooks so this is more about the relative difference.)

[lizadaly]

Looking into that failing test; it's passing locally.

[matteocargnelutti]

This is excellent, @lizadaly .

Add annotations using insertAdjacentHTML rather than insertAdjacentElement (Mozilla says it's faster and my benchmarks agreed)

I would have assumed the opposite!