manager: allow searching for deleted dossier

Deleted dossiers don't appear in the unfiltered list though. You have to
know its id to find it.

app/dashboards/dossier_dashboard.rb

@@ -14,6 +14,7 @@ class DossierDashboard < Administrate::BaseDashboard
     text_summary: Field::String.with_options(searchable: false),
     created_at: Field::DateTime,
     updated_at: Field::DateTime,
+    hidden_at: Field::DateTime,
     types_de_champ: TypesDeChampCollectionField,
   }.freeze
 
@@ -38,6 +39,7 @@ class DossierDashboard < Administrate::BaseDashboard
     :types_de_champ,
     :created_at,
     :updated_at,
+    :hidden_at
   ].freeze
 
   # FORM_ATTRIBUTES