Added code to generate the full path to the image preview

The image entity can't generate the full path as it doesn't know the location of the web directory.
Added tests for ConfigureEvent, ConfigureQueryEvent and PersistenceEvent
Admin minor CS fixes
Add blocks before and after main navigation

This commit adds two blocks before and after main navigation.

I will use this to add :

- locale switcher for admin
- store switcher
.....
Allow to use translation_domain in FormMapper fieldDescriptionOptions

This will call this function https://github.com/sonata-project/SonataAdminBundle/blob/master/Admin/BaseFieldDescription.php#L466, so it works even if translation_domain is not specified.
Add body_attributes block

This allow us to customize login screen
+ change admin design based on user for company which have different brands
Merge pull request #1682 from EmmanuelVella/patch-11

Allow to specify a translation_domain option in list, show and filter "add" method
Merge pull request #1777 from nicolas-bastien/patch-1

Add blocks before and after main navigation
Updated italian translation
Merge pull request #1784 from andreadelfino/translation-it

Updated italian translation
Merge pull request #1773 from pulzarraider/event_test

Added tests for ConfigureEvent, ConfigureQueryEvent and PersistenceEvent
Merge pull request #1775 from pulzarraider/admin_cs_fix

Admin minor CS fixes
Merge pull request #1708 from katalisha/master

Added code to generate the full path to the image preview
Merge branch 'sonata/master'