Implement redirects in ItemContent

This allows ItemContent to be instantiated based on an
EntityRedirect, and allows WikiPageEntityLookup to load
such ItemContent objects.

This does not yet provide a way to create or resolve
redirects in the database.

Bug: 57744
Change-Id: If091211c1d1d9ab62dd04f23ec6df6cc5f3ffebb
Merge "Implement redirects in ItemContent"