This introduces common CSS for defining tab navigation for content.

It's meant to be generic enough to be used in a variety of cases, but

also to be extendable for additional presentation modes going forward.
Tabs are defined by creating a ul.rb-c-tabs element, with

li.rb-c-tabs__tab elements inside of it. Each has a label, which can

optionally have a short version for mobile devices.
On mobile devices, tabs are also horizontally scrollable, keeping the

tabs from wrapping around awkwardly.
Mixin functions are available to customize tab presentation.

.set-color-scheme() defines the colors used by the tabs, and

.set-flush-with-border() makes the tabs flush with a border of a

content container.
Tabs make use of another new CSS addition, color schemes. We constantly

define colors throughout our UI, rarely making use of centralized

variables for them. In an effort to both standardize colors and to make

it easy to apply them, the beginnings of a set of namespaced color

definitions and pre-defined color schemes (grouping together

backgrounds, borders, etc.) now exist in colors.less. Color schemes

can be passed currently to .set-color_scheme(), and other functions

down the road, to give a standard way of customizing components.

Made use of this with the Issue Summary Table.
Tested the full and short labels in mobile/desktop modes, and tested

the horizontal scrolling in mobile mode.
Tested the mixins in different conditions.