User search: Once again, change uri handling.

They can indeed be non-unique.

priv/repo/migrations/20200930082320_user_ur_is_index_part_three.exs

@@ -0,0 +1,8 @@
+defmodule Pleroma.Repo.Migrations.UserURIsIndexPartThree do
+  use Ecto.Migration
+
+  def change do
+    drop_if_exists(unique_index(:users, :uri))
+    create_if_not_exists(index(:users, :uri))
+  end
+end