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.)
The following imply that it is a secret and therefore each user should have a different one, does everything still work with different values ?

http://stackoverflow.com/questions/5697962/rails-authlogic-persistence-token-question
http://jeremycook.ca/2010/03/28/creating-a-persistent-login-mechanism/