Add K6 overrides and more advanced theming system

[snide]

By providing a K6 theme over the K7 components, we're hoping to enable Kibana engineers to consume this framework today, while retaining a consistent look-and-feel in the Kibana UI.

Guidelines

1. Only override SCSS variables.
2. If necessary, abstract parts of KUI behind variables and mixins to enable No Elasticsearch assets SVG #1.
3. Avoid overriding selectors and styles directly as much as possible. Only do this as a last resort.

K6 theme

The K6 theme consists of:

1. Changing colors to match the palette.
2. Changing font-size to match.
3. Changing some spacing to match, which generally just means making spacing tighter.

[cjcenizal]

One comment. This PR needs a rebase and a note in the CHANGELOG that we've adding two new themes, and then I think we're good to merge!

[cjcenizal]

Can we delete this line?

[snide]

@cjcenizal I rearranged how we import sass in regards to themes. Wanted to contain themes to their own directory vs having the files spread out in each folder that needed it. Also went ahead and wrote up docs for how to make themes. Give it another look, then we should be OK to merge.

[cjcenizal]

LGTM! Let's merge this and cut a new release!