$wgMathValidModes and orthogonal MathJax enabler

To adjust the selectable math rendering modes in user preferences $wgMathValidModes
is introduced.

* $wgUseLaTeXML becomes unnecessary use $wgMathValidModes[] = MW_MATH_LATEXML;
  to enable the LaTeXML rendering mode
* add $wgMathValidModes[] = MW_MATH_MATHJAX; to enable MathJax

Currently, MathJax is bound to the MW_MATH_SOURCE rendering mode.
This change makes the base mode user configurable.
That means before MathJax is loaded the png fallback image could
be displayed.

ATTENTION: This change modifies global variables as follows:
* MW_MATH_MATHJAX is deprecated.

Bug: 57981
Change-Id: Ibf705cb66754d04e4c7eafd1e98608b25d7dbb94