(Bug 69827): Supported nested <ref> tags once more

* Nested <ref> tag support was broken in 5ee1733d since that patch
  effectively processed <refs> only on the final DOM rather than
  extracted <ref> information in sub-pipelines. In doing so, it
  broke support for <ref> in <ref> tags which are supported by {{efn}}
  templates and used as follows.

  {{efn|A clarification.{{sfn|Smith|2009|p=2}}}}

  The {{sfn}} tpl generates a <ref> tag inside another <ref> tag
  generated by the {{efn}} tpl.

* This patch fixes the breakage by processing <ref> content after it
  is extracted (in case it is known to have nested <ref> tags).

* This fixes the rendering on enwiki/Otto_I%2C_Holy_Roman_Emperor
  and is an improvement over when it was broken. The nested <ref>
  gets id 18 (just like the Cite.php handling) whereas it used to
  get id 1 before the breakage.

* TODO:
  - Figure out a way to add a test for this.

Change-Id: Ib82b75f66249b2133a123adbe6fd7acbfd8ec8fb