attempt to detect apiHost and session, show host+login input window if needed

[themightychris]

Create an optional singleton or controller for the emergence-apikit package that provides an optional stock UI on top of the existing apikit classes.

Any existing use of Ext.Msg in the API connection classes should be removed in favor of events, such that none of the data classes have any dependency on framework components outside of the data package.

The controller could provide error/retry UI as well as a method applications could use called requireAuthentication that takes a callback that will be run after an API host has been connected to and authenticated against.

apiHost would be determined from, in priority order:

1. page query parameter
2. set to null if window.SiteEnvironment is defined
3. included as field is login prompt to user

session would be loaded from, in priority order:

1. window.SiteEnvironment if apiHost is null
2. API.getSessionData(...)

SlateAdmin can then wrap its viewport initialization and/or defer its routing until requireAuthentication executes its callback