Support fallback languages for icons

Use mw.language.getFallbackLanguageChain() to get the
fallback language chain. This chain currently always contains 'en'
as last element. Remove the last element to prevent a fallback to 'en'.

Reimplement autoIconOrOffset to correctly prefer sprite before icon.

Bug: T87247
Change-Id: I452dd45d20ea4dd542d63274b7aad0272e20ea12