Bug 939174 - Launch whats new FTU tutorial to demo edge gestures etc.

* Add shared VersionHelper
* Unit tests for VersionHelper, FTULauncher
* Check and set device.previous_os in FTULauncher/FTU app
* Launch full ftu when ftu.enabled always
* Launch 'whats new' ftu tutorial when upgrade detected
* Add config for 1.3-2.0 and 1.4-2.0 upgrade tutorial
I would move all these logic to UIManager, but we could do it in a followup. Could you open a bug for removing the management of `classList` outside `UIManager`? Thanks!