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
Why dont we follow the same as https://github.com/mozilla-b2g/gaia/pull/20087/files#diff-2ef37288baccfded46aad14297ac7949R222 ? Actually the code is quite similar, could we merge both into a function?