This CL adds a button that collapses values list of a block.
Button is displayed for every block with non-empty values.

Change-Id: I4b65af81e25349f38341df487d42698c9d006a00
Reviewed-on: https://go-review.googlesource.com/c/144557
Run-TryBot: Yury Smolsky <yury@smolsky.by>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>