Refactor Mobilizon.Federation.ActivityPub and add typespecs

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

lib/graphql/resolvers/activity.ex

@@ -5,6 +5,7 @@ defmodule Mobilizon.GraphQL.Resolvers.Activity do
 
   import Mobilizon.Users.Guards
   alias Mobilizon.{Activities, Actors}
+  alias Mobilizon.Activities.Activity
   alias Mobilizon.Actors.Actor
   alias Mobilizon.Service.Activity.Utils
   alias Mobilizon.Storage.Page
@@ -12,6 +13,8 @@ defmodule Mobilizon.GraphQL.Resolvers.Activity do
 
   require Logger
 
+  @spec group_activity(Actor.t(), map(), Absinthe.Resolution.t()) ::
+          {:ok, Page.t(Activity.t())} | {:error, :unauthorized | :unauthenticated}
   def group_activity(%Actor{type: :Group, id: group_id}, %{page: page, limit: limit} = args, %{
         context: %{current_user: %User{role: role}, current_actor: %Actor{id: actor_id}}
       }) do