Apply the new styleguide - update the sidebar

[jenweber]

Work should be done starting from this branch: https://github.com/ember-learn/cli-guides/pull/141

The actual commits should likely be made to ember-learn/guidemaker-ember-template, or there may be CSS that just needs to be removed from this repo.

The goal is to get the sidebar using the colors from the Ember Styleguide https://github.com/ember-learn/ember-styleguide, have passable typography, and generally look in harmony with the redesign. If an ember-styleguide component is available, please use it instead of following the points below.

Problems to fix

• [x] As of opening this issue, the sidebar items are too bold. They should be less-bold, and then the active menu item should be the boldest thing.
• [x] The blue leading box that appears on hover does not match the color palette
• [ ] Anything looking strange on the Ember Guides side

[jayjayjpg]

Might it make sense to also add a point as part of this issue on updating the width of the outline on focus / hover? I think it'd be nice to display it full-width instead of having it cut off to the side - what do you think?