Add "cover" and "semilarge" thumb sizes

[towerofnix]

Development:

• Resolves Consider adding 600x600 "cover" thumbnail #244.
• Based on Pre-crop images as squares for smaller thumbnails #243.
• This is a draft PR for discussion below, as well as for an implementation detail (selectively loading thumbs based on display DPI, if possible via HTML alone). Expecting to return to this a bit later.

Both 600x600, but cover is square-cropping and semilarge is not. cover is intended for use in generateCoverArtwork of course (i.e. the main art image on album, track, flash pages and such). semilarge is a corresponding smaller "right-fit" available for the client-side large image preview.

The motivator for this change is that the 400x400 medium thumbs currently used for cover artworks result in mildly blurry upscaling on high-DPI displays in the wide layout, as well as on about 450–600px wide displays of even normal CSS-matches-display-pixel DPI in the thin layout.

A 400x400 image doesn't exactly look awful rendered at about 600x600, though, or even when upscaled to about 800x800 on high-DPI displays in the thin layout. From some unscientific sampling, 600x600 at quality 94 results in about 40% larger files than 400x400 at quality 95. That's a moderately large tradeoff for mobile and slower network access, and you can always opt in to getting the best-fit size on particular covers just by clicking and showing the client-side image preview.

I've left this as a draft, besides the uncertainty, because I want to investigate if it's possible to restrict loading higher resolution images to high-DPI devices purely in HTML (no JavaScript nonsense).