Introduce activity filters

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

lib/graphql/schema/actors/group.ex

@@ -153,6 +153,7 @@ defmodule Mobilizon.GraphQL.Schema.Actors.GroupType do
       arg(:limit, :integer, default_value: 10, description: "The limit of activity items per page")
 
       arg(:type, :activity_type, description: "Filter by type of activity")
+      arg(:author, :activity_author, description: "Filter by activity author")
       resolve(&Activity.group_activity/3)
       description("The group activity")
     end