Add toggle-style checkbox component to styleguide

Fixes: DES-259

Test plan:
- Go to /styleguide and click on the Toggle link
- Test out the toggle for usability and accessibilty
- It should support keyboard navigation and a
  disabled state added to the input element

Change-Id: I5823969a8b2195230fa19445f61004ac1300143d
Reviewed-on: https://gerrit.instructure.com/40857
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Colleen Palmer <colleen@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Chris Hart <chart@instructure.com>