Bug fix on "book buy" JS code when a book title contains quotes
Our BookOperationListener now implements EventSubscriberInterface
At last! The user language is correctly managed :-)

* Added a UserLanguageProvider data provider: it provides the user language, based on app available languages and request accepted languages
* Added a "alina_reyes_ebooks_store.available_languages" parameter
* Removed the ugly "locale" Twig global hard-coded variable, replaced with a "user_language" global variable linked to the UserLanguageProvider data
* Removed the hard-coded or misused user language usage here and there, replaced with the use of  the UserLanguageProvider
* Added a UserLanguageListener, which sets the Request locale from the UserLanguageProvider data
* the usual code style fixes...