Bug 53243: Accept prefix-only interwiki / language links

Also adds two Parsoid-only tests that uncover broken serialization support for
new language / interwiki links. We should probably match absolute URLs against
the list of patterns and serialize to the locally defined interwiki / language
prefix when that is defined. See bug 53432.

Change-Id: I60c2219a955194a37397d00d8c6468f626129ea8
Merge "Bug 53243: Accept prefix-only interwiki / language links"