Make <ref>s nested in <references> editable.

* Track <ref>s birthed in <references> tags and capture their
  HTML and set <references>.data-mw.body.html to the concatenated
  HTML for all those child <ref> tags.

* No change in parser-tests because our test normalization code
  strips data-mw before comparing results.  Since this patch
  only changes data-mw output for "References: 5. ..." test,
  there is no change.  In any case, this patch updates the
  data-mw output for the test in question so that if we do
  modify our normalization routines, the test will still pass.

Change-Id: Iaea752dec03e5e7f0d36baf3b7a9973f1b85e8a5
Merge "Make <ref>s nested in <references> editable."