(Bug 48311): Added missing barrier to newline migration

* refs leave behind a marker in place and cannot use the
  mw:Object/* meta-types since DOM processors expects matched
  start/end pairs.  So, mw:Ext/Ref/Content meta-marker slips
  through the conditions that prevent newline migration across
  the ref-tags.

* Added a new check (in migrateTrailingNLs DOM pass) for extension
  placeholder markers since these are also newline migration barriers.

* Fixes parsing of echo "a <ref/>\n<ref/>" so that the newline
  between the two ref-tags is not migrated out of the paragraph.

Change-Id: I9c10ab734bc4af29fdf94ad06a664271a315b7cb