(bug 42160) Add basic implementation of interwiki links

Amends by gwicke:

* Added languages to base config and interwiki list.

* Used vsrc to pass source of link targets to the link handler. This fixes
  stuff like entity-encoded link targets.

* Shadowed href with original source, which should already provide basic
  editing support.

* Refactored the serializer handling for interwiki / language links a bit

Overall one more wt2html, 6 more wt2wt, and 64 more selser parser tests are
passing.

Change-Id: Ia3f1ee67693d7f60bed386a3f2709fcb4dcbf550
Merge "(bug 42160) Add basic implementation of interwiki links"