Refactor githubAuthButton as a reactive widget

[micahjsmith]

Currently githubAuthButton is a ToolbarButton. Then we call the authCallback function at an interval to determine if the user has successfully authenticated, and if so, to change the button visually to indicate success (i.e. background color to green). I'm sure it would be better to implement this as a ReactWidget where the button itself is responsible for determining the authentication status and changing its own appearance accordingly.