Add persistence_token for users

Attempting to run fixture-loading tests locally gives this error:

```Mysql2::Error: Field 'persistence_token' doesn't have a default value: INSERT INTO `users` (`id`, `login`, `email`, `activated_at`, `created_at`, `updated_at`) VALUES (1, 'testuser', 'test@example.com', '2009-12-12 21:08:07', '2014-08-22 05:36:52', '2014-08-22 05:36:52') (ActiveRecord::StatementInvalid)
./features/step_definitions/fixtures_steps.rb:5:in `/^I have loaded the fixtures$/'```

This adds a persistence_token so the tests will run. (This finishes what https://github.com/otwcode/otwarchive/pull/1574 started.)
Loading fixtures works whether it's the same or different because -- at least as far as I can tell -- the persistence token in the cuke fixtures isn't doing anything more than acting as a placeholder so the users can be created. (We don't have any cukes for the remember me function. I could try testing it manually by changing the values in the fixtures we use for seeding, too, but I'd prefer not to reseed my database at the moment.)