Show message for unknown notifications

Previously it would just show an infinite spinner for GitHubbers with access to unreleased features. This just updates it to show this message with a link to the existing notifications UI:

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![github_notifications](https://cloud.githubusercontent.com/assets/173/3488081/1292ef6e-04c7-11e4-9be7-3201a774ed16.png)

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/cc @kansaichris