Merge pull request #135 from fredericbonjour/Admin

Updated Backoffice
Fixed loading when tree is empty.
Fixed JS error when scope.preSubmit() does not return a promise.
Fixed loading when tree is empty.
Added attribute "hide-icons" on tree-view directive.
Added "root-node-label" attribute to set the label of the root node.
Moved Website-related styles to the Website plugin.
Removed unused controller reference.
Fixed DocumentPickerMultiple Twig macro.