Click a user's nickname to initiate a call

[Related story](https://trello.com/card/as-a-user-i-see-and-can-click-on-another-person-s-nick-to-initiate-a-call-with-them/5152a164b33d31f85c002c70/17)

- added `UserEntryView`
- added user icons in connected people list
- added events for sign in & sign out
- added new `CallView` to display the user's video stream

Live demo: http://talkilla-n1k0.herokuapp.com/

r=@dmose