refactor(SidePanel): API update and fixes

[anicholls]

Summary

SidePanel needs a changes to bring it in line with our API & Pattern Guidelines:

• Change sidePanelBackgroundColor to backgroundColor
• Not Breaking: Change open param in onBreakpointChange to aboveBreakpoint. We shouldn't suggest the behavior - consumers can do what they want with the return value.
• Bonus: There were lots of undefined styles being applied based on conditionals in the styled components. Note: use undefined instead of '' in emotion ternaries.

Closes #105.

	Visual	Code
Breaking Changes	No	Yes

Code

• sidePanelBackgroundColor prop has been renamed to backgroundColor.

Checklist

• branch has been rebased on the latest master commit
• tests are changed or added
• yarn test passes
• all (dev)dependencies that the module needs is added to its package.json
• code has been documented and, if applicable, usage described in README.md
• module has been added to canvas-kit-react and/or canvas-kit-css universal modules, if
  applicable
• design approved final implementation
• a11y approved final implementation
• code adheres to the API & Pattern guidelines