Fix checkmarks to render correctly

# Problem

Right now a list of checkmarks in GitHub-flavored markdown, that normally render like this (on GitHub):

```
- [ ] Check item 1
- [ ] Check item 2
```

...instead render like `• [x]`.

Although we don't want to make these into working check items on our end (yet, at least), we do want to render them correctly.

We can do this by doing string parsing in the Ember app itself, rather than in the markdown rendering step on the API.

從 Ember 應用程式的 Markdown 渲染路徑開始，重現 issue 中 GitHub-flavored checklist 的範例，並將其與 API 的 Markdown 輸出進行比較。追蹤清單變成 `• [x]` 的位置；完成的標準是 checklist 標記能正確渲染，同時維持不可互動，且不變更 API 的 Markdown 渲染步驟。

由索引模型根據 Issue 內容生成。