content/static/html: use a single link for licenses
content/static/html: use a single link for licenses

The header is updated so that a single link to the license tab is shown,
instead of a separate anchor link to each license.

Change-Id: Ib6b2d948f96227805e73816c0cc1108eea1ca8a5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/259213
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite