This new checkbox component uses a plain `input` component internally,
so the browser treats it like a native checkbox. It is styled by hiding
the native checkbox and replacing it with Font Awesome icons (the same
that we are using in Figma).
Support for a 'disabled' state and an indeterminate state has been
added as well. The `onClick` prop has been made optional, as it may not
be required if the parent component is intercepting the click instead.
The `regular` Font Awesome font style needed to be added so that we
could use the `far fa-square` icon for the unchecked checkbox.
Mark Stacey
b628ff05d1