Rename some global variables that violate the naming convention

* $wg(.*) variables from extension should start with the extension name
    i.e. for extension Math $wgMath(.*)
* But keep $wgUseMathJax, because it has been used for 2 years now.
* Add release notes

Change-Id: Ib70a9f7767890cc4618bc10c2610784f5b17e670
Merge "Rename some global variables that violate the naming convention"