Signup Data Form: Collect/display why_signed up from user

[aaronschachter]

Blocked by #1903

If the signup data form is configured to collect why the user signed up, add it as a form element into the dosomething_signup_user_signup_data_form and save to the signup entity's why_signedup property.

Also, display the why_signedup value when viewing a signup entity.