Split mediawiki.special.preferences into separate file for each function

This JavaScript code does four things:
* Convert a simple form into one with tab navigation
* Convert a simple message box into pretty notifications
* Prevent browser window from being closed with unsaved changes to preferences
* Add some enhancements to the timezone field

I will be making some large changes here for T117781 and having
everything mixed in one file was making that difficult. I avoided
making any cleanup changes to make it easier to review this change.

Change-Id: I5e31a661177002e40708614011c1f7e08ec4f44d