[release-branch.go1.7] godoc/dl: clean up display of stable versions

Under "Stable Versions", only show the latest minor versions of the most two
recent major versions.

Under each of those versions, move the downloads for primary ports to
the top of the table and the other downloads under a separate
"Other Ports" heading.

Add a listing for every stable Go version under "All versions",
collapsed by default.

Fixes golang/go#17574.
Updates golang/go#17018.

Change-Id: I7d74fef1b44a319a4bf65dedf24ffb27a7009f60
Reviewed-on: https://go-review.googlesource.com/34824
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/35095
Reviewed-by: Chris Broadfoot <cbro@golang.org>
3 files changed