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