Switch to photon-icons check image for checkbox when available

[lmorchard]

The checkbox component is using a local image for the check icon. We should switch to a shared icon from photon icons when available.

[nt1m]

@lmorchard According to https://github.com/FirefoxUX/icons/issues/79#issuecomment-403063753 , the checkbox on the icons website is the right one: https://design.firefox.com/icons/viewer/#check

[bwinton]

Really? Huh. Someone should update the Design System, then! 🙂