Correction de la liaison User -> Group (auparavant ManyToMany, maintenant ManyToOne) + scission des voters.
Merge branch 'master' of git.dedicated-panel.net:dedicated-panel/dedicated-panel

Conflicts:
	src/DP/Core/MachineBundle/Form/MachineEntityType.php
	src/DP/Core/MachineBundle/Voter/MachineAssignableVoter.php
	src/DP/Core/UserBundle/Entity/User.php