[META] Use component-based styling in CSS

While developing websites with Drupal, it's common to get really complex selectors (especially with menu styling.)

There are 2 reasons for this:

1. We are overriding CSS rules in core that are already complex.
2. We have no easy classes to match exactly what we want.

While reading about the SMACSS technique, the class naming technique made a lot of sense. It uses modular class names. Related components get class names that include both the component name and the variant name.

.COMPONENT
.COMPONENT--VARIANT
.COMPONENT__SUB-ELEMENT
.COMPONENT--VARIANT__SUB-ELEMENT

For example, instead of ul li.expanded we should just use .menu__item-expanded.

Sub-issues:

#1713168: Add SMACSS-style classes to header
#1713170: Add SMACSS-style classes to menus
#1713172: Add SMACSS-style classes to tabs
#1713320: Add SMACSS-style classes to messages
#1714986: Add SMACSS-style rulesets to pager
#1719726: Simplify various selectors to use SMACSS-style classes