feat(Actions): Add Actions component

[markdalgleish]

This PR adds the initial Actions component (formerly known as ButtonGroup), for rendering a responsive group of buttons/links at the bottom of a form/card/etc.

This PR also makes the following changes:

• Introduces a responsive flexDirection prop to Box, e.g. <Box flexDirection={['column', 'row']}>
• Fixes a bug with none values for margin and padding. The corresponding values in the CSS were incorrectly set to none rather than 0.

Example usage

<Actions>
  <Button weight="strong">Submit</Button>
  <TextLink href="#">Cancel</TextLink>
<Actions>

Mobile

Desktop

Development Notes

Internally, we've made the following changes.

• Moved box-sizing: border-box from Box into our CSS reset, so that link elements rendered by TextLinkRenderer use the correct box model.
• Introduced a new private component called Overlay to centralise rules for absolutely positioned cover elements, as well as a FieldOverlay component to centralise form overlay rules. This was done to ensure that the focus styles are consistent between TextLinkRenderer and Button, but the idea is that should expand this pattern to other fields going forward.
• Limited all transition atoms to transform and opacity.

[michaeltaranto]

Thoughts on hoisting this before the ActionsConsumer? If it's inline it has no contextual behaviour so can return fast.

[markdalgleish]

Ah yeah, nice idea 👍

[michaeltaranto]

Any reason for having an array as the third parameter here, rather than flat parameters?

[markdalgleish]

More good ideas 👍

[michaeltaranto]

💪