choosealicense.org has HSTS due to #483.

It would be great to go all the way and preload HSTS: https://hstspreload.org/?domain=choosealicense.org

Steps would be:

• Serve an HSTS header with a value of Strict-Transport-Security: max-age=63072000; includeSubDomains; preload
• Make sure choosealicense.org can be accessed from curl/golang? (this times out from HQ).
• Submit to https://hstspreload.org/?domain=choosealicense.org

@parkr: could you help me find where the HSTS header served by choosealicense.org is configured?