Bug 939174 - Launch whats new FTU tutorial. r=borjasalguero,alive

* Add shared VersionHelper
* Unit tests for VersionHelper, FTULauncher
* Check device.previous_os in FTULauncher/FTU app
* Launch full ftu when ftu.enabled always
* Launch ftu whats new tutorial when upgrade detected
* Add config for 1.3-2.0 upgrade tutorial
* Address review comments