Introduce the group activity section

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/graphql/schema/resource.ex

@@ -9,6 +9,7 @@ defmodule Mobilizon.GraphQL.Schema.ResourceType do
 
   @desc "A resource"
   object :resource do
+    interfaces([:activity_object])
     field(:id, :id, description: "The resource's ID")
     field(:title, :string, description: "The resource's title")
     field(:summary, :string, description: "The resource's summary")