Protect MathML from Tidy

MediaWiki installations that use the setting
$wgUseTidy = true; are unable to output
MathML since the well defined MathML elements
are filtered out by Tidy. This was reported as
  http://sourceforge.net/p/tidy/patches/84/ .

This change hides MathML blocks from
Tidy.

Bug: 66516
Change-Id: Ib48b91238c3eddd6a86b62f6ce57801d7058f0d8