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Plus, prevent instant rejection of promises in the `noEarlierThan` helper

The purpose of the component is to give visual feedback to the user
when a press on a button starts a long-running process
that can succeed or fail.

* Design review changes - 2

Also:
- change the black colour across the site, and add soft-black colour
- update the Checkbox component to change cursor appearance to pointer
  when mousing over label, and to catalogue classNames props
  that can be passed to the Checkbox.
- update the Button component so that the active state of the PrimaryButton and
   SecondaryButton have a 1px-wide border (to avoid changes in button size
   when toggling between disabled and enabled states of the button)
- add option to prettier config to remove trailing commas from objects

- Add PrimaryButton
- Add SecondaryButton
- Add tests and stories for these buttons