Promote LivePreview from its experimental state

* Now loads modules that are required to view the page
* Make use of the API
* Removed the old deprecated LivePreview events
* Fixes additional margin in changes mode

Much of this is inspired by the userscripts ajaxPreview by the users
AlexSm and Anomie.

As a next step, I would like to add mw.template support to this.

Bug: 24134
Bug: 53327
Change-Id: Ia339f43122cfe801913628475c7db88d037ec269
Merge "Promote LivePreview from its experimental state"