talksapp: Enforce HTTPS

Add redirects from HTTP to HTTPS. Add HSTS header. Update example URLs
to use HTTPS.

Fixes golang/gddo#536

Change-Id: Iffbf1cff8ed0e87a8cae90496907577f410e4444
Reviewed-on: https://go-review.googlesource.com/93276
Reviewed-by: Tuo Shan <shantuo@google.com>