Add push, the web server for Skia Push.

Note that this CL adds duplicate files for:

  res/imp/login.html
  res/imp/scaffold.html

That duplication will be cleaned up in a follow-up CL.

BUG=skia:
TBR=mtklein

Review URL: https://codereview.chromium.org/794723002
Add current and status URL subparts

BUG=skia:3154

Review URL: https://codereview.chromium.org/799613003
Setup scripts for skiapush instance.

This CL also includes the setup script to build skia-pushable-base, which is used as the boot disk for GCE images that participate with Skia Push.

BUG=skia:
R=rmistry@google.com

Review URL: https://codereview.chromium.org/794243003