Delay the page ready until all formulas are fully displayed

The formulas are displayed asynchronously on page load the MathJax macro should use the xwiki-page-ready module to delay the page ready until all formulas are displayed. See XWIKI-19266 and:

• http://docs.mathjax.org/en/latest/web/configuration.html#performing-actions-after-typesetting
• https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/FrontendResources/Page%20Ready/

Without this the client-side PDF export might not include the complete formulas.