Toggle - cornerstone toggle improvements

Summary

This PR can be summarized in the following changelog entry:

adds a switch to RTL to all the demo pages in the standalone app
updates the dir attribute on the <html> element when switching to RTL, this partially addresses #665
adds Toggle and CornerstoneToggle demos to the "UI Controls" tab (formerly "Checkbox" tab) in the demo pages
Toggle: removes the <label> element and uses aria-labelledby instead
Toggle: passes the ID as a prop with a default value
CornerstoneToggle: changes the text to "Mark as cornerstone content" (addresses design feedback)
Toggle: uses getRtlStyle() for RTL languages and adjusts various CSS
Toggle: prevents the page from scrolling when using the Spacebar
Toggle: when using the Spacebar, switch the toggle on keyup instead of keydown, as that's the native behavior
Toggle: makes onToggleDisabled() work when using the Spacebar
Toggle: uses aria-disabled when the Toggle is disabled

This PR can be tested by following these steps:

Also, please verify the style adjustments from #682 are OK.

Fixes #681