Update the mailer preview class to use a block statement method

[Chocksy]

Here is an example of a mailer_preview that would be ideal for our code from now on. This would remove the duplicate code:

class InvitePreview < ActionMailer::Preview
  def accepted
    show_email do
      @mail = InviteMailer.accepted(@user, @organization, true).message
    end
  end

  protected

  def show_email(&block)
    @mail = nil
    Organization.transaction do
      @user = User.create!(name: 'Bob User', email: '[email protected]')

      @invite = Invite.create(user: @user, email: '[email protected]')

      yield

      raise ActiveRecord::Rollback
    end
    @mail
  end
end

That's an example code adapt it to our preview classes.