Calibre-Web: fix cover/title thumbnails for PDF's and other docs/books

[holta]

Tracking ticket to make sure this gets into Internet-in-a-Box (IIAB) 7.0

Thanks to @tim-moody & @OzzieIsaacs who are tracking this down together @ janeczku/calibre-web#827

[holta]

Stretch Goal torwards serving all teachers' needs:

• Can thumbnails and/or metadata be made easily visible from community photos/podcasts/videos, moving beyond just PDF's/docs ? Whether or not this includes PowerPoint documents?
• Do Nextcloud and Moodle already show file thumbnails/previews as such?
• Might there be a WordPress plugin that does somewhat similar? @ericnitschke @kananigit
• Interoperability with Android clients should be a given? Can IIAB recommend/include an Android filesharing app for offline communities? #1541

[holta]

I'm assuming this is now fixed, by PR #1565

[holta]

PR #1567 looks good.

Please continue testing that thumbnail preview / cover pages appear for PDF's at http://box/books on Raspbian, Debian 9/10 and Ubuntu.