Create fake users and roles.

- Add method create_default_users in MockData module
  to create fake users and roles.

- Add rake task create_default_users in app:setup namespace.
  create_default_users calls method create_default_users to
  create fake users and roles.

- Call create_default_users rake task from db:test:prepare rake task.
Refine create_default_users method.

- Refine create_default_users method to stuff fake students.
Fix feature tests.

- Fix feature tests to work with standard set of fake.