Add support for GraphQL handling of group follows

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

priv/repo/migrations/20211022093530_add_notify_to_followers.exs

@@ -0,0 +1,9 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddNotifyToFollowers do
+  use Ecto.Migration
+
+  def change do
+    alter table(:followers) do
+      add(:notify, :boolean, default: true)
+    end
+  end
+end