FIX: Broken preview for single math equations

Previously we would remove elements from the DOM, this caused Ember to
get extremely confused and blow up its rendering.

Instead only hide the element, do not remove it.

assets/javascripts/initializers/discourse-math-mathjax.js.es6

@@ -85,7 +85,7 @@ function decorate(elem, isPreview) {
         // don't bother processing previews removed from DOM
         if (elem.parentElement && elem.parentElement.offsetParent !== null) {
           window.MathJax.Hub.Typeset($math[0], () => {
-            $elem.remove();
+            $elem.hide();
             $mathWrapper.show();
           });
         }