- l10n: Add Chinese (simplified) extension locales (Thanks, @enlove!)

- Linting (ESLint): Avoid reporting `no-console`
- Refactoring: Avoid Firefox console errors with hidden iframe
- Docs (CHANGES): Fix Markdown heading level

editor/extensions/ext-locale/mathjax/zh-CN.js

@@ -0,0 +1,8 @@
+export default {
+  name: '数学',
+  buttons: [
+    {
+      title: '添加数学计算'
+    }
+  ]
+};