HTML reader: Revise treatment of li with id attribute.

Previously we always added an empty div before the list
item, but this created problems with spacing in tight
lists.  Now we do this:

If the list item contents begin with a Plain block,
we modify the Plain block by adding a Span around
its contents.

Otherwise, we add a Div around the contents of the
list item (instead of adding an empty Div to the
beginning, as before).

Closes #3596.

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