Set href in url/ext links in transcluded content.

* A bug report on wikitext-l about href not being set on url links
  found in refs when transcluded via templates.

* In turns out the LinkHandler code had a bug where href was not
  being set on ext and url links when the links came from template
  content.

* Other minor cleanup.

* No change in test results. 

Change-Id: I42f9b8683b1048b61e4e63947d6740efd253363b