Double-indented checkboxes aren't rendered

### Expected
If the checkbox is rendered as a checkbox in the editor, it should look like checkbox in the preview too.

### Observed
Hitting `Tab` twice (double-indent) for a new checklist item will break the rendering in preview:

https://user-images.githubusercontent.com/73365754/123971545-d4159880-d9c2-11eb-814b-74a0bc77dc3f.mov

### Reproduced
0. Open a new note and enable markdown
1. Start creating a new checklist
2. For a new item, hit `Tab` once ~> the item is at level 2 now. It's shown correctly in preview (Sub-bug: When on Windows, I experienced that first `Tab` hit will sometimes act as double-tab. This is seen in the video above for the case of outdent - it jumps back to level1 from level3, instead of jumping to level2).
3. Hit `Tab` once more for this checkbox ~> it jumps to level3, and not rendered correctly in the preview

### Where did you see the bug
- OS version: Windows 10 Home | Parallels on MBP 16"
- Simplenote app version: desktop 2.14.0-beta1

Start by reproducing the issue in the markdown editor and preview using the numbered checklist steps, especially a checkbox indented to level 3. Compare how level-1 and level-2 checkboxes render with the double-indented case. Done means nested checkboxes remain visible as checkboxes in preview, including the reported Windows tab and outdent behavior.

Written by the indexing model from the issue text.