reveal.js writer and template: reuse mathjax URL...

...provided by the argument to `--mathjax` or the normal
pandoc default, rather than a hard-coded one in the template.

Closes #4701.

data/templates/default.revealjs

@@ -237,7 +237,7 @@ $if(maxScale)$
 $endif$
 $if(mathjax)$
         math: {
-          mathjax: 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js',
+          mathjax: '$mathjaxurl$',
           config: 'TeX-AMS_HTML-full',
           tex2jax: {
             inlineMath: [['\\(','\\)']],

src/Text/Pandoc/Writers/HTML.hs

@@ -296,10 +296,11 @@ pandocToHtml opts (Pandoc meta blocks) = do
                   (if stMath st
                       then defField "math" (renderHtml' math)
                       else id) $
-                  defField "mathjax"
                   (case writerHTMLMathMethod opts of
-                            MathJax _ -> True
-                            _         -> False) $
+                        MathJax u -> defField "mathjax" True .
+                                     defField "mathjaxurl"
+                                       (takeWhile (/='?') u)
+                        _         -> defField "mathjax" False) $
                   defField "quotes" (stQuotes st) $
                   -- for backwards compatibility we populate toc
                   -- with the contents of the toc, rather than a