Implement visual context for cards, tables and lists

As part of PF Milestone 3 (cards, tables, lists) we will implement the visual context for them.
Basically, it means adding the grey background in all pages, and the white background in headers of the same pages.

Official PF Demo:
https://www.patternfly.org/v4/documentation/core/demos/page-full?component=Page%20component%20default%20nav%20header%20example

to do:

• [x] proof of concept to see if it's doable in 1 week: https://github.com/3scale/porta/pull/1182
• [x] check with Amy if this is ok direction
• [ ] dry up most common rule sets in extends/mixins
• [ ] sweat the details with hocus-pocus css