fix(math): restore formula overflow scroll for MathJax 4 (#2770)

Since the MathJax 4 upgrade (97a537e), the `mjx-container` overflow
rules were dropped, so long display equations extend past the viewport
and cause page-level horizontal scrolling on mobile (the fix from #545
for #543 was lost). MathJax 4 defaults to `displayOverflow: 'overflow'`;
set it to 'scroll' so long equations scroll within their own container.

assets/js/data/mathjax.js

@@ -21,5 +21,9 @@ MathJax = {
     ],
     {%- comment -%} equation numbering {%- endcomment -%}
     tags: 'ams'
+  },
+  output: {
+    {%- comment -%} scroll long display equations instead of overflowing the page {%- endcomment -%}
+    displayOverflow: 'scroll'
   }
 };