gddo-server: add UI to turn on/off redirect to pkg.go.dev

This change adds UI for the option to set a cookie and have
candidate godoc.org links redirect to their equivalent pkg.go.dev
counterparts. If a user returns to godoc.org with the utm_source
form value equal to 'backtogodoc', it will display an option to
turn off the automatic redirect.

The intention is to have a link on pkg.go.dev that allows the
user to return to godoc.org and also turn off the automatic redirect
without having to know the special URL param values.

Updates golang/go#37099

Change-Id: I27bd03cbc484a1e504795ff669224c2a2a7d72b6
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/222315
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
5 files changed