content: remove scheme from iframe URLs

This prevents mixed content errors preventing iframes with HTTP URLs
from loading on the blog site, which is served by HTTPS.

Fixes golang/go#15683

Change-Id: I28cff3f7838f780ed05524ed620a851ef5bf4e28
Reviewed-on: https://go-review.googlesource.com/23180
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
11 files changed