Vary marker style of unordered lists (<li> in <ul>) by level of nesting

[gnprice]

This is something that browsers do unless instructed with CSS otherwise, and that Zulip web/desktop and the zulip-mobile RN app (which uses a webview for the message list) therefore get automatically. It's subtle, but would be good to match.

For the web behavior, see:
https://html.spec.whatwg.org/multipage/rendering.html#lists
https://www.w3.org/TR/css-counter-styles-3/#simple-symbolic

Here's a screenshot of how that comes out:

Note there are three different styles for the outer three levels, and then the third style repeats for further levels.

The markup for that test message is:

* a
  * b
    * c
      * d
      * d2
    * c2
  * b2
* a2

Also note there is no such variation for ordered lists <ol>. Those are by default always marked with 1, 2, 3, etc., regardless of nesting level.