Add description slug field for events

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

priv/repo/migrations/20190411124138_event-add-description-slug.exs

@@ -0,0 +1,9 @@
+defmodule Elixir.Mobilizon.Repo.Migrations.EventAddDescriptionSlug do
+  use Ecto.Migration
+
+  def change do
+    alter table(:events) do
+      add(:slug, :string)
+    end
+  end
+end