Postgres migration to add allowed_event_props field to site (#2894)

* add migration

* add not null constraint

* remove not null constraint and default value

priv/repo/migrations/20230503094245_add_event_prop_allowlist_to_site.exs

@@ -0,0 +1,9 @@
+defmodule Plausible.Repo.Migrations.AddEventPropAllowlistToSite do
+  use Ecto.Migration
+
+  def change do
+    alter table("sites") do
+      add :allowed_event_props, {:array, :string}
+    end
+  end
+end