Update list item newline handling to follow Parsoid's model

Update list item newline handling to follow Parsoid's model

This improves on commit 34bd573144883af9ace7ead32d3be8f692431718 by matching
Parsoid's newline handling in the PHP parser. It is the outcome of a
discussion with Erwin, where we agreed that

* foo
* bar

should produce

<ul><li>foo</li>
<li>bar</li></ul>

See the discussion in https://gerrit.wikimedia.org/r/#/c/94443/

The original rendering issue this tried to address is no longer present after
a change to the template. The pure CSS solution is now working.

Bug: 39617
Bug: 56809
Change-Id: Ib7aa9449bbd994cb23b83b3f23cff944b1cddadf