Divide the seeders for production and testing

[troccoli]

The current seeders populate the database with testing data: users, seasons, clubs, etc.

However, part of that data is not actually test data. For example the Site Administrator role should always be added.

We should include in DatabaseSeeder only the seeder for actual production data. Then, create another seeder, TestSeeder or something, to add the test data.

Then the process would be

• php artisan migrate:fresh --seed to create a production ready database
• php artisan db:seed --class TestSeeder to add testing data