[x/tour] tour: redirect HTTP traffic to HTTPS
[x/tour] tour: redirect HTTP traffic to HTTPS

Set secure to always on all handlers so that HTTP traffic
is redirected to an HTTPS URL with the same path.

References:

•	https://cloud.google.com/appengine/docs/standard/go/application-security#https_requests
•	https://cloud.google.com/appengine/docs/standard/go/config/appref#handlers_secure

For golang/go#42281.

Change-Id: Ic03e01d5858e1e5b8ae1b523ab34d970e1403ce0
Reviewed-on: https://go-review.googlesource.com/c/tour/+/266817
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
X-Tour-Commit: 59417492f03e0d55e544815ff163973cef919f67

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website