Use revealjs's math plugin for mathjax.

This is a thin wrapper around mathjax that makes math look better
on revealjs.

See https://github.com/hakimel/reveal.js/#mathjax

We do this by setting the 'mathjax' boolean variable and
using it in the revealjs template. Also, for revealjs
and mathjax, we don't assign the usual thing to the 'math'
variable, since it's handled by mathjax config.

Closes #3743.

data/templates/default.revealjs

@@ -229,15 +229,24 @@ $endif$
 $if(maxScale)$
         maxScale: $maxScale$,
 $endif$
+$if(mathjax)$
+        math: {
+          mathjax: 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js',
+          config: 'TeX-AMS_HTML-full',
+        },
+$endif$
 
         // Optional reveal.js plugins
         dependencies: [
           { src: '$revealjs-url$/lib/js/classList.js', condition: function() { return !document.body.classList; } },
           { src: '$revealjs-url$/plugin/zoom-js/zoom.js', async: true },
-    $if(notes-server)$
+$if(notes-server)$
           { src: '$revealjs-url$/socket.io/socker.io.js', async: true },
           { src: '$revealjs-url$/plugin/notes-server/client.js', async: true },
-    $endif$
+$endif$
+$if(mathjax)$
+          { src: '$revealjs-url$/plugin/math/math.js', async: true },
+$endif$
           { src: '$revealjs-url$/plugin/notes/notes.js', async: true }
         ]
       });

src/Text/Pandoc/Writers/HTML.hs

@@ -253,7 +253,9 @@ pandocToHtml opts (Pandoc meta blocks) = do
                          H.script ! A.src (toValue url)
                                   ! A.type_ "text/javascript"
                                   $ mempty
-                      MathJax url ->
+                      MathJax url
+                        | slideVariant /= RevealJsSlides ->
+                        -- mathjax is handled via a special plugin in revealjs
                          H.script ! A.src (toValue url)
                                   ! A.type_ "text/javascript"
                                   $ case slideVariant of
@@ -285,6 +287,10 @@ 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) $
                   defField "quotes" (stQuotes st) $
                   maybe id (defField "toc" . renderHtml') toc $
                   defField "author-meta" authsMeta $