Add support for new release radar tracking

Ignore Low and Undefined blueprints in the active blueprints list,
unless they are already Implemented.
Adjust colors for Essential/High/Medium/Low accordingly.
Fix alignment of past milestones table.

Change-Id: I33c0058b6f827d67340cc8f6077bfa81f07782a2
Reviewed-on: https://review.openstack.org/36202
Approved: Thierry Carrez <thierry@openstack.org>
Reviewed-by: Thierry Carrez <thierry@openstack.org>
Tested-by: Jenkins