Added "xdebug.max_nesting_level=200" to the Dockerfile setup

This should fix Symfony's Web Profiler errors... :-)
Added an email sending on ebooks downloads

* Added a BookDownloadEvent
* Added an email sending for this event in our BookOperationListener, with its own new parameters and Twig template
* A little bit of factorization in the BookOperationListener
* Added some comments in parameters.yml.dist