no issue
- moved `document-title` Route extension's functionality into the `ui` service
- updates the title each time the router service emits a route changed event
- `ui.updateDocumentTitle()` can now be called directly from components rather than the confusing `this.send('updateDocumentTitle')` bubbling behaviour
- refactored the `titleToken` implementation to use the now-formalised `RouteInfo`'s `metadata` field (https://github.com/emberjs/rfcs/blob/master/text/0398-RouteInfo-Metadata.md#appendix-a)
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