Improve and activate groups

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

priv/repo/migrations/20200928150725_add_contacts_to_events.exs

@@ -0,0 +1,10 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddContactsToEvents do
+  use Ecto.Migration
+
+  def change do
+    create table(:event_contacts, primary_key: false) do
+      add(:event_id, references(:events, on_delete: :delete_all), primary_key: true)
+      add(:actor_id, references(:actors, on_delete: :delete_all), primary_key: true)
+    end
+  end
+end