Remove the last remaining use of the `$rootScope` service to broadcast /
subscribe to the `OAUTH_TOKENS_CHANGED` event.

Instead the `auth` service is now an `EventEmitter` and the `session`
service subscribes directly to `oauthTokensChanged` events from it.
Whereas we replaced most other events with observers which react to data
changing in the store, this particular use case doesn't lend itself to
that and it makes more sense for the `auth` service to emit events to
notify interested consumers.