manager: localize dossier enums

In the previous commit, we cleaned up the localization of the
Dossier.state enum. This prevented administrate from crashing, but now
the dossier enum are no longer localized in the Manager.

By using a `Field::Enum` type, we instruct the administrate plugin
to look up our localized name for the Dossier state enum.

app/dashboards/dossier_dashboard.rb

@@ -10,7 +10,7 @@ class DossierDashboard < Administrate::BaseDashboard
   ATTRIBUTE_TYPES = {
     id: Field::Number.with_options(searchable: true),
     procedure: Field::HasOne,
-    state: Field::String,
+    state: Field::Enum,
     user: Field::BelongsTo,
     text_summary: Field::String.with_options(searchable: false),
     created_at: Field::DateTime,