Epub: refresh global Twig variable 'edition' to allow templates to pick 'edition.id' value

Twig template variable `edition.id` is assigned in assembleBook() but it was not available in template `content.opf.twig` because `edition` was assigned previously as a global Twig variable that was not being refreshed.

This PR refreshes the value of the global `edition` Twig variable just after `edition.id`  gets assigned.