doc: hide video and share if being served from CN

In the case where requests are coming from mainland China, hide
links to locations that are blocked and functionality that is
not permitted.

Additionally, some very small cleanup of the JS.

This change requires https://go-review.googlesource.com/c/52873

Change-Id: I7fc68748e629dbe5b966d6bf117e7f7b546966eb
Reviewed-on: https://go-review.googlesource.com/52872


Reviewed-by: Chris Broadfoot <cbro@golang.org>