Convert Dropdown to ES6 class

[interactivellama]

Prepare the way for #844 and #1089

Additional description

This removes the last mixin from non-deprecated components. (menu-picklist still has one, but Combobox should be used now instead of that).

In order to do this I just copied, parts of the mix-in into menu-dropdown (thus created duplicate code. This PR also breaks apart the menu navigating mixin so part of it can be used if so desired by #1089.

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• TravisCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
• Tests have been added for new props to prevent regressions in the future. See readme.
• Review the appropriate Storybook stories. Open http://localhost:9001/.
• The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
• Review tests are passing in the browser. Open http://localhost:8001/.
• Review markup conforms to SLDS by looking at DOM snapshot strings.
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[kevinparkerson]

Why do we define these above the class then reference those variables here, rather than define them inline here only once? Is that just a DSR standard?

[interactivellama]

Yeah, artifact of history--but also from my point of view, even though we have a website, there are still engineers who don't trust docs and look at the source code for prop types, so putting them first helps their dev experience.

[kevinparkerson]

From what I can tell this seems just fine. My only comment is more just a question in regards to formatting vs anything that needs correction.