gddo-server/assets: add Black Lives Matter banner Change-Id: I14308b23539760cd54577daabc7617eb40c75e6e Reviewed-on: https://go-review.googlesource.com/c/gddo/+/237601 Run-TryBot: Andrew Bonventre <andybons@golang.org> Reviewed-by: Julie Qiu <julie@golang.org>