Reused per-track artwork isn't handled very nicely

[towerofnix]

We hash the contents of track covers to avoid saving multiple copies of the same file (and to early exit out of downloads, at least in principle). This appears to work as intended, but it leaves tracks with duplicate artworks totally unaccounted for, so you have to manually cross-reference the appropriate cover yourself.

It'd be nicer if we symlinked/aliased towards the file we did save, instead, that way you can quickly identify the originals (in a file browser / terminal), and we'd still use minimal space/networking.

Good test album: https://nuclear333.bandcamp.com/album/inhospitable-delisted