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