CloseButton

[shawnbot]

This adds the CloseButton component to close #114, which I think means just the little ✖️ that lives in the corner of various modals and menus. We'll probably need to refactor this (and perhaps Button, too) a bit more as we put it to use in other components (for instance, SelectMenu in #116), but I think this is a good start.

I had to add support for a new value of the Button component's scheme prop: scheme="octicon", which translates to the btn-link text-inherit utility classes. I originally looked at using Box-btn-octicon, but that didn't work unless it was contained by a Box.

We could revisit this to use the btn-octicon class if we decide to take on primer/css#413.

[shawnbot]

We should call out the onClick prop here so that it's easier for people looking at the code to see that it's supported.

[shawnbot]

We agreed in the component review that this is probably not the right way do the close button, because this particular component shouldn't support most of the props that Button does (children, disabled, linkStyle, etc.). It's also saddling the Button component with features that only a "subclass" uses, which has a pretty nasty code smell.

Instead, let's introduce an OcticonButton that renders a <button> with a required aria-label and an Octicon child, and can easily be used to make a close button.