UIShell components: rename "isOpen" prop to "open"

[weaseldotro]

The isOpen prop should be renamed to open in the UIShell components: HeaderAction, HamburgerMenu, SideNav.

This would be a breaking change for apps currently using these components, but would improve consistency in prop naming with the rest of the library. There are only 3 components using isOpen, while 11 other components use open.