There's no Settings->Agreement path for PolyGerrit users, but if we
link directly to the page in the instructions, Gerrit will inform them
that they can access the page by switching to the old UI.

Fixes #20207

Change-Id: I0887ee854e4ac5975b5f305adb6259b81b41618f
Reviewed-on: https://go-review.googlesource.com/42412Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>