Bug 1017314: Load tutorial config in a loadConfig method. r=borjasalguero

* Start tutorial preload in ftu UIManager.init
* Create per-layout tutorial config file, use l10nKey as title property name in config
* Refactor Tutorial to loadConfig/ensureConfig and update tests accordingly
* Remove unused data-maxstep attributes.
Merge pull request #19807 from sfoster/configurable-tutorial

Bug 1017314: Load tutorial config in a preload method. r=borjasalguero