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
Merge "Update list item newline handling to follow Parsoid's model"