Separate Mathoid from the Math extension

Mathoid is supposed to be run at a server separated from
the MediaWiki math extension. It should use the MathJax
version customized to the MediaWiki needs as submodule.
It has been moved to the new repository
mediawiki/services/mathoid

Change-Id: I00b42926ba91043570c5058e0352c0e743a400b0