<Button /> should support <a /> and <Link /> functionality

[jessepinho]

Currently, the docs I wrote for <Button /> incorrectly state that, to use it as a link, you can simply wrap it inside an <a /> or <Link /> tag/component. However, this is invalid HTML, and results in an issue @VanishMax discovered where users must hit Tab twice for each button wrapped in a link.

To fix this, the <Button /> component should be rewritten to be able to take the form of a link. We'll have to be thoughtful about this, since we don't only want to support <a />, but also any type of other link component that libraries like React Router use for links (in React Router's case, the <Link /> component).