app: use https instead of protocol-relative URLs
app: use https instead of protocol-relative URLs

Protocol-relative URLs (//blog.golang.org/...) are redundant, we use
HTTPS everywhere.
Use the https:// scheme instead.

Updates golang/go#17961.

Change-Id: I710959584971f1fa155bab3ea623c33a9f6ab9e9
Reviewed-on: https://go-review.googlesource.com/37884
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/playground,golang/playground,golang/playground