gddo-server/assets: update banner

Banner is updated to have a separator in the middle, which looks better
for centered content per UXD.

Desktop:
https://photos.app.goo.gl/5ZkmLqi8RCDRRPc69

Mobile:
https://photos.app.goo.gl/Lf8aw5yWuaB6kGe68

Change-Id: I635e9a19133f66d7a58192ae70f21dd0913b2901
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/278412
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
2 files changed