add flags to permissions groups

Flags can be added to permissions group to store some metadata.

An interface `PermissionsGroupFlagProvider` allow to add some flag to the form which create/
edit permissions groups.

Resources/migrations/Version20180905101426.php

@@ -0,0 +1,25 @@
+<?php declare(strict_types=1);
+
+namespace Application\Migrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20180905101426 extends AbstractMigration
+{
+    public function up(Schema $schema) : void
+    {
+        $this->addSql('ALTER TABLE permission_groups ADD flags JSONB DEFAULT \'[]\' NOT NULL');
+        $this->addSql('ALTER TABLE group_centers ALTER permissionsgroup_id DROP NOT NULL');
+
+    }
+
+    public function down(Schema $schema) : void
+    {
+        $this->addSql('ALTER TABLE permission_groups DROP COLUMN flags');
+        $this->addSql('ALTER TABLE group_centers ALTER permissionsgroup_id SET DEFAULT NULL'); 
+    }
+}