This replaces the static Language methods getFallbackFor(),
getFallbacksFor(), and getFallbacksIncludingSiteLanguage(). There is
100% unit and integration test coverage for the new class.
One deliberate functional change: I changed one place where we threw
MWException to InvalidArgumentException.
Bug: T201405
Depends-On: Ie7a89f6ed7d52a0bc01672019ff92e7ee105a1f3
Change-Id: I49222eb55f1feec5b1dcd40f364cffe0c8801855
