Feature: Sync custom column read state with kobo

[NotSimone]

Currently, the read state when books are synced with Kobo is not correctly synced when the "Link Read/Unread Status to Calibre Column" option is used.

This change makes it so that changing read state on either Calibre or the Kobo e-reader should sync the read state correctly. Note that the Kobo reading state has a third state - reading - that cannot be represented in the Calibre column and so gets ignored.

This also adds an additional configuration option "User for Calibre Read Column" so that only the specified user's syncing will update or be updated by the column in Calibre. By default, it is configured to be the Admin user (id 1).

resolves #2740

[NotSimone]

I should mention a caveat - it seems that modifying the custom column in calibre does not change the last_modified timestamp (or anything else vaguely resembling a timestamp as far as I could tell), which means you cannot reliably sync a change in read state from Calibre to Kobo.

Fortunately, the other direction is fairly reliable and that's the direction most of us care about anyway ;)

[quizzmaster]

I merged this PR for myself and it works like a charm :)

[NotSimone]

I've just rebased this onto the tip of develop

[sgtio]

This would be a great thing to have! I was going crazy wondering why Calibre web was not setting the read column in my Calibre library after I finished a book in my Kobo. But if I got it right, the custom column is only written to if you read a book through Calibre web? Or is it only read by Calibre web, but never written to?

Anyway, any hope that this PR can be merged?

[NotSimone]

The custom column is already being updated properly when the read state is updated through the web UI. This change only affects the column state when it is synced against a Kobo e-reader.