Nested bullets render inconsistently; only support two levels of nesting

[codebykat]

A user in #3185157-zen requested help with this so I was experimenting. I found that:

• It's possible to use spaces to create a nested list on iOS
• Markdown preview does not always correctly render nested lists
• Lists with more than two levels of nesting are collapsed
• HTML lists are stripped out (they work in web/Electron and publish)

Related: #274

Screenshots

[charliescheer]

Adding some testing text for this issue

* one
* Two
  * three
  *  four
      *five
      *six

<ul><li>hi</li>
<li><ul><li>two</li></ul></li></il>