The notification manager has been refactored to use the extension
platform module instead of using `extensionizer` directly. The
extension platform API presents a more ergonomic API, and it correctly
handles errors (which the old notification manager did not). Methods
that the extension platform lacked have been added.
It has been updated to use `async/await` instead of callbacks as well,
for readability.
The `triggerUI` function has also been updated to use the extension
platform instead of `extensionizer`.
