Add indexes to group activities

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

priv/repo/migrations/20210310084210_add_indexes_to_activities.exs

@@ -0,0 +1,8 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddIndexesToActivities do
+  use Ecto.Migration
+
+  def change do
+    create(index(:activities, [:group_id], name: "activity_group_id"))
+    create(index(:activities, [:author_id, :type], name: "activity_filter"))
+  end
+end