Allow to add metadata to an event

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

priv/repo/migrations/20210805142745_add_metadata_for_events.exs

@@ -0,0 +1,9 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddMetadataForEvents do
+  use Ecto.Migration
+
+  def change do
+    alter table(:events) do
+      add(:metadata, :map)
+    end
+  end
+end