Message: Don't include Title objects in the serialization (part 1)

Message: Don't include Title objects in the serialization (part 1)

Stringify the title instead. This will help avoid running into the
PHP/HHVM serialization incompatibility described in T210528.

This is part 1: start stringifying and using the string, but continue
storing the Title object for the benefit of mixed environments where
some servers still only handle a Title object.

Bug: T210528
Change-Id: I07aac3aab2d4e27a7203f4e4fb3ce1b5d86c517c