Call notifications

[Related story](https://trello.com/card/as-a-user-after-i-ve-initiated-a-call-to-another-person-talkilla-should-signal-the-call-to-their-computer/5152a164b33d31f85c002c70/50)

- basic impl. of call notification (WiP)
- close ws connection on page unload
- user is automatically signed out on page unload
- notification models & views
- live incoming call notification
- dedicated ws events for notifications
- better error notifications for sign{in,out}
- added tests for incoming call offer
- moved signin/signout test helpers to the root scope
- call accept/deny events
- update users list when a ws connection is closed
- clear notifications on signout

r=@dmose as we already discussed about this patch