Add toast base component #42
Description
@intern implement this; it should appear as a toast notification at the top of the screen.
needs to have props to control colour/intent (e.g. default, success, error), and ideally uses existing component system libraries. if not, make a recommendation for a toast library that would work.
Ideally, any component should be able to call a global function showToast(foo) that then displays a toast, without the need to drill a toast component or provider down to the call site. toasts should be stacked on top of each other if more than one is visible at a time.