Make `tabindex=0` optional

As per F2F.

We will add a menu option to switch off tabindex=0 on MathJax output. This will allow users to disable it should they want to.

The Menu structure will be: Math Settings => Renderer => checkbox "Include in tab order" (below the AssistiveMML checkbox)

The docs will strongly discourage developers from initially disabling this (by means of configuration) as it will render the menu inaccessible.