CYB-161  https://jira.ets.berkeley.edu/jira/browse/CYB-161

* This change references to the current user's canvas ID to current_user.canvas_id
* This change should also set up a way to easily extend, and add on other attributes of the current user by modifying ApplicationController#current
* Also adds current_user.user_name data (provided when the app is launched from canvas)

https://jira.ets.berkeley.edu/jira/browse/CYB-161