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

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* Simplenote 2.14
* User report: 4225592-zen

Start by reproducing the issue in Simplenote 2.14: add a Markdown list with more than five nested levels and open preview mode. Inspect the preview path that renders nested lists; done means level-six content, bullet text, and all deeper levels are visible as they are in the web app.

Written by the indexing model from the issue text.