Handle error when no authed user tries to access NC. github.com/shurcooL/notificationsapp#2

Notification Center app requires an authenticated user. Currently, if you try access its URL directly (it's not linked anywhere, but someone can type it directly in browser):

https://src.sourcegraph.com/.notifications

(You can open that link in an incognito window to reproduce.)

You get:

template: notifications.html.tmpl:6:29: executing "notifications.html.tmpl" at <.RepoNotifications>: error calling RepoNotifications: permission denied

Instead, it's better for the app to return unauthenticated HTTP status, which will have Sourcegraph redirect you to a login screen. This is the best way to handle resources that require auth when you access them directly and you're not logged in.

Fixed in 39752e62.