Add logout button that stops all peerconnections, restyle settings

* Update settings menu style: add 1 logout button, display image (if available), name, connected network, add FAQ link
* Removed logout buttons for individual social networks, as we will only be supporting 1 network at a time for web store launch, and there won't be any way to get back the connect buttons (networks.html) until you log out via settings.
* Changed logout in uProxy core to stop any peerconnections.  Previously after logout these peer connections would remain open, with no way to close them (because the Network object would have been removed)
* Change UI loggedIn logic to use a ui.onlineNetwork field.  Binding to a function (loggedIn) wasn't re-evaluating when necessary and didn't give us any information about which network we are logged into (currently only 1 supported).
* Fixed bug when sending USER_SELF message to UI (had been calling flushQueuedInstanceMessages which is no longer implemented and threw a silent error).  Now we can get the name and image of the logged in user.

Tested in grunt test, with new tests for syncNetwork_ in ui
I just made a change for this in core.ts and social.ts.  Now in the Network's login method, I am sending a uProxy.Update.NETWORK to the UI after login completes but before the onceLoggedIn_ promise is fulfilled - I also removed the ui.UpdateAll from core.ts after calling login as it was redundant.  These changes make sure that the UI will know that the network is logged in before any user updates are sent to the UI (those are all delayed until after network.onceLoggedIn_ fulfills).

Also I'm thinking for a later pull request it would be good to have 2 arrays in the code: one just containing the available network names (strings), and a second containing each logged in network object (would be empty if you aren't logged in, or could have multiple entries for the same network name once that is supported).