Adhere to Apache WEBSITE NAVIGATION LINKS POLICY

Correct Apache Project Website Checks identified with:
https://whimsy.apache.org/site/project/madlib

The following were previously reported as a critical issue:

 - Events
 - License
 - Thanks
 - Security
 - Sponsorhsip
 - Privacy

For reference, here are the Website Navigation Links Policy we should
adhere to:

https://www.apache.org/foundation/marks/pmcs.html#navigation

This commit includes:

* Add "ASF Links" navbar menuitem with the following text and links

  - ASF Homepage - https://www.apache.org
  - Apache Events - https://www.apache.org/events/current-event.html
  - Code of Conduct - https://www.apache.org/foundation/policies/conduct.html
  - Privacy Policy - https://privacy.apache.org/policies/privacy-policy-public.html
  - Security - https://www.apache.org/security/
  - Sponsorship - https://www.apache.org/foundation/sponsorship.html

* Update "footer" of all main pages to include

  text: Licensed under the Apache License, Version 2.0.
  link: https://www.apache.org/licenses/LICENSE-2.0