Revert "Remove useless VPNConfigurationInline class"

This reverts commit 3a2135ecf5bcb291fe59be552a377b728f8111e8.

This is actually needed when editing a subscription.

coin/vpn/admin.py

@@ -7,6 +7,12 @@ from coin.configuration.admin import ConfigurationAdminFormMixin, IPSubnetInline
 from coin.utils import delete_selected
 
 
+class VPNConfigurationInline(admin.StackedInline):
+    model = VPNConfiguration
+    # fk_name = 'offersubscription'
+    readonly_fields = ['configuration_ptr']
+
+
 class VPNConfigurationAdmin(ConfigurationAdminFormMixin, PolymorphicChildModelAdmin):
     base_model = VPNConfiguration
     list_display = ('offersubscription', 'activated', 'login',