Missing content on nested lists with more than 5 levels

In preview mode, nested list with more than 5 levels are missing all content starting on the 6th level.

### Expected
To see all lists as I do on the web app version:

### Observed
Nothing past the 5th level is visible:

* 6th level: bullet points are preserved but there lines are empty
* 7th level and further: missing

### Reproduced

1. Add a markdown list with more than 5 levels
2. Preview the note

***

* Simplenote 2.19-rc-1 (139)
* User report: 4225592-zen

Reproduce the problem in preview mode with a Markdown list deeper than five levels, following the steps in the issue. Trace the preview rendering path and compare it with the web app behavior; done means content and list markers remain visible at the sixth level and beyond.

Written by the indexing model from the issue text.