Introduce Clearance::UsersConteroller#user_params

This makes it easier to overwrite `user_params` when needing to provide extra
fields during signup.

For example, with `strong_parameter`:
```ruby
class UsersController < Clearance::UsersController
  private

  def user_params
    params.require(:user).permit(:email, :password, :first_name, :last_name)
  end
end
```