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>

talksapp/app.yaml

diff --git a/talksapp/app.yaml b/talksapp/app.yaml
index 193596f..86c8242 100644
--- a/talksapp/app.yaml
+++ b/talksapp/app.yaml
@@ -7,16 +7,21 @@
 - url: /robots\.txt
   static_files: assets/robots.txt
   upload: assets/robots.txt
+  secure: always
 - url: /favicon\.ico
   static_files: present/static/favicon.ico
   upload: present/static/favicon.ico
+  secure: always
 - url: /static
   static_dir: present/static
+  secure: always
 - url: /play\.js
   static_files: present/play.js
   upload: present/play.js
+  secure: always
 - url: /.*
   script: _go_app
+  secure: always
 
 env_variables:
   CONTACT_EMAIL: ''        # set contact email for /-/bot.html

talksapp/assets/home.article

diff --git a/talksapp/assets/home.article b/talksapp/assets/home.article
index e10d0f6..3dbab6a 100644
--- a/talksapp/assets/home.article
+++ b/talksapp/assets/home.article
@@ -13,8 +13,8 @@
 
 The syntax for URLs is:
 
-    http://go-talks.appspot.com/github.com/owner/project/file.ext
-    http://go-talks.appspot.com/github.com/owner/project/sub/directory/file.ext
+    https://go-talks.appspot.com/github.com/owner/project/file.ext
+    https://go-talks.appspot.com/github.com/owner/project/sub/directory/file.ext
 
 The supported file extensions (.ext) are .slide and .article.