Reuse existing addresses when creating an event

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

priv/repo/migrations/20190821161658_add_origin_id_to_addresses.exs

@@ -0,0 +1,11 @@
+defmodule Mobilizon.Repo.Migrations.AddOriginIdToAddresses do
+  use Ecto.Migration
+
+  def change do
+    alter table(:addresses) do
+      add(:origin_id, :string)
+    end
+
+    create(index(:addresses, [:origin_id], unique: true))
+  end
+end