XWIKI-8744: Introduce per wiki distribution status
* Refactor distribution.vm so that it is independent of the available steps.
* Allow steps to perform some action before moving to the next step.
* Allow steps to handle AJAX requests.
* Refactor welcome step so that it is independent of the available steps.
* Refactor the velocity code from default UI step (split into smaller Velocity macros).
* Implement the Upgrade Mode step.
* Use wiki syntax in outdated extensions step and remove deprecated calls to $msg.get()