Lose article content when trying to edit two articles at the same time

Open two articles in two tabs. Edit the first article, switch to markdown mode, then also edit the second article and switch to markdown mode. Return to the first article and save, then go to the second article and hit the save button. The save process will get stuck and the content of the second article gets lost. Also the home page of ZeroBlog misfunctions and can't scroll down, and it shows that the second article has 3 comments from post 23.
Quite dangerous, reproducible with new clone of ZeroBlog.
I check the data/data.json and find that `body` column is missing, if I add it back and sign the blog, the home page becomes normal again.
Tested with Tor browser.

重現雙分頁編輯流程，然後檢查 data/data.json 以及涉及 save、markdown-mode 和 home-page 的進入點。完成的標準是儲存第二篇文章不再卡住或遺失內容，而且首頁能夠在留言資料正確的情況下正常捲動。

由索引模型根據 Issue 內容生成。