Revise ebook instructions to address Calibre problem

[joewiz]

An email to the ebooks mailbox reported a problem opening the frus1941v01 epub in iBooks on macOS. Research into this revealed that the ebook opens fine on iBooks on iOS and in Murasaki on macOS, and the oddity about this volume, compared to others that open fine in iBooks is that it contains some Calibre metadata in the OEBPS/content.opf file. This appears to be added when opening an epub file in Calibre. We should probably remove any instructions which say to open an epub file in Calibre, such as in https://github.com/HistoryAtState/hsg-project/wiki/Publishing-New-Volumes-to-Website#create-ebooks. The original ebook instructions at https://github.com/HistoryAtState/hsg-project/wiki/ebooks could be a model. It shouldn't be necessary to open epubs or mobis in Calibre at all; the ebook batch helper produces the mobis. Any time we open an epub or mobi in an external ereader for proofing we should do this on a copy of the file; and only the original files should be uploaded to S3.