change to multiple page structure

refactor:

* multiple page js [reference] (http://stackoverflow.com/questions/11674824/how-to-use-requirejs-build-profile-r-js-in-a-multi-page-project/11730147#11730147)
* use navigate instead of set currentPanel
* rename `PanelUtils` to `PanelHandlers`
* refactor to use `navigate` instead of `set currentPanel`
* refactor default panel id to UPPER_CASE constant