Banner screenshot job

Renders a banner using PhantomJS, for every campaign language, and store the
images to disk.

TODO:
* Correct high-load behavior by waiting for page load or aborting.
* Add a switch to toggle between in-context rendering and clipping to div#centralNotice.
* Debug empty files.
* Null file if size is zero.

Change-Id: Ia7dcdaea11fddf5d2746e7e06e58b5bc9b857147
Merge "Banner screenshot job"