Hygiene: clean up overlay header related code

* Make upload progress look right again.
* Use #f3f3f3 as .overlay-bottom background (used in old header and new
  last edited).
* Reuse general overlay template in KeepGoingOverlay.
* Remove .with-margins class from templates (no longer exists).

Change-Id: I0054f3214716244981075ef51603c3267002ef20
Merge "Hygiene: clean up overlay header related code"