WIP on books order fine-tuning

* Added a "price" column to our "BookPayment" entities; that way, event if the price of a book is modified, the transaction price is archived
* Better "book order" email
* Added "admin.language" & "admin.timezone" to "alina_reyes_ebooks_store" parameters
* Added a BookOrderOperationEvent class
* "Book" entity fix: price is now annotated as a float instead of a string
Removed useless comments in previous DB migrations
Removed unused "AlinaReyesEbooksStore" Configuration init