Add additional interwiki links as requested in various bugs

* Add "mul" for Wikisource (bug 32189) and Wikiversity
* Add aliases for wikis using non-standard ISO codes (bug 21915), for the
  benefit of Wiktionary and for forward compatibility when bug 19986 is
  fixed

Bug: 16962
Bug: 21915
Change-Id: Iaba783c48c563d52479e0e830e29cacb0cd75d0a