Deprecate client-side logIn

[jmileham]

The JS client is designed for client-side usage. Login doesn't need to be part of the JS client because a proper integration of TestTrack with an authenticated app will involve a server-side client like test_track_rails_client as well. That server side library will provide an updated visitor ID to the JS client upon successful login and all subsequent JS requests should use it.

This may mean refreshing JS client state after an authentication API call to use the new visitor ID in single-page apps.