dev#90497 : purge unused files in specific directory

FEATURES/90497

+        '90497' =>
+            ['Label' => $this->_('Administration : Outils de purge du dossier userfiles'),
+             'Desc' => $this->_('Purge des fichiers non utilisés dans les articles'),
+             'Image' => '',
+             'Video' => '',
+             'Category' => 'Administration',
+             'Right' => function($feature_description, $user) {return true;},
+             'Wiki' => 'http://wiki.bokeh-library-portal.org/index.php?title=Explorateur_de_fichiers',
+             'Test' => '',
+             'Date' => '2019-06-17'],
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