Fix React duplicate key warning

[eerohele]

If a notification is rendered more than once simultaneously, React throws a warning like this:

react-dom.inc.js:18238 Warning: flattenChildren(...): Encountered two children with the same key, `...`. Child keys must be unique; when two children share a key, only the first child will be used.

Using the index of the item instead of the options map as the key fixes the issue. It should be safe here since a notification component is never re-rendered.

Further testing demonstrated the statement above false. Using the index as the key caused the notifications to get messed up in situations where multiple different notifications were rendered simultaneously.

Generating a UUID for each notification seems to work better.