feat: implement markdown file preview in Ace editor (#3431)

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Co-authored-by: Oleg Lobanov <oleg@lobanov.me>

frontend/src/i18n/en.json

@@ -24,6 +24,7 @@
     "ok": "OK",
     "permalink": "Get Permanent Link",
     "previous": "Previous",
+    "preview": "Preview",
     "publish": "Publish",
     "rename": "Rename",
     "replace": "Replace",

frontend/src/i18n/zh-cn.json

@@ -22,6 +22,7 @@
     "ok": "确定",
     "permalink": "获取永久链接",
     "previous": "上一个",
+    "preview": "预览",
     "publish": "发布",
     "rename": "重命名",
     "replace": "替换",