require markdown-it-mathjax only if MATHJAX env var exists

resource/js/util/markdown-it/mathjax.js

@@ -1,11 +1,16 @@
-export default class MathJaxConfigurer {
-
-  constructor(crowi) {
-    this.crowi = crowi;
-  }
-
-  configure(md) {
-    md.use(require('markdown-it-mathjax')());
-  }
-
-}
+export default class MathJaxConfigurer {
+
+  constructor(crowi) {
+    this.crowi = crowi;
+
+    const config = crowi.getConfig();
+    this.isEnabled = !!config.env.MATHJAX;  // convert to boolean
+  }
+
+  configure(md) {
+    if (this.isEnabled) {
+      md.use(require('markdown-it-mathjax')());
+    }
+  }
+
+}