Per the designs, we want the button on the top left of the screen to be 'Cancel'. This diff introduces the `headerLeft` component.
I followed some of the work for the invite links modal on `native` to get the types correct for creating a separate component for `headerLeft` and `headerRight` (later in the stack), so also adding @tomek as a non-blocking reviewer for these 3 diffs
Depends on D8157