Bug 1038630 - Split Tutorial init into init, start methods.

* init early in FTU, call start from 'Get started' button
* Dont wait to loadConfig in ftu AppManager
* Add 1.3..2.1 and 1.4..2.1 upgrade steps for easier testing on master
* Remove unused ensureConfig
* Fix tutorial init/start logic in FTU AppManager
* Add jsdocs for Tutorial
* Remove surplus Tutorial.init call from UIManager
* Update FTU UIManager mock
Merge pull request #22274 from sfoster/bug-1038630-tutorial-redux

 Bug 1038630 - Split Tutorial init into init, start methods. r=fcampo