Allow callback functions for creating SpecialPages.

This enables factory functions to be registered for special
pages, as an alterative to giving a class name. This follows the
same rationale as Ieb85493a7765, which introduced factory functions
for API modules.

Change-Id: Ia2107dc5af7869187ba5dc02a1bef46d6801e138
Merge "Allow callback functions for creating SpecialPages."