refactor: server code (#2545)

+ Remove duplicated await
+ Replace some legacy codes with ES6
+ Fix some of eslint problems

server/modules/rendering/markdown-core/underline.js

@@ -1,12 +1,12 @@
 const renderEm = (tokens, idx, opts, env, slf) => {
-  const token = tokens[idx];
+  const token = tokens[idx]
   if (token.markup === '_') {
-    token.tag = 'u';
+    token.tag = 'u'
   }
-  return slf.renderToken(tokens, idx, opts);
+  return slf.renderToken(tokens, idx, opts)
 }
 
 module.exports = (md) => {
-  md.renderer.rules.em_open = renderEm;
-  md.renderer.rules.em_close = renderEm;
+  md.renderer.rules.em_open = renderEm
+  md.renderer.rules.em_close = renderEm
 }