Add an unique index on addresses url

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

priv/repo/migrations/20210422155913_add_index_to_addresses.exs

@@ -0,0 +1,11 @@
+defmodule Mobilizon.Storage.Repo.Migrations.AddIndexToAddresses do
+  use Ecto.Migration
+
+  def up do
+    create_if_not_exists(unique_index("addresses", [:url]))
+  end
+
+  def down do
+    drop_if_exists(index("addresses", [:url]))
+  end
+end