MovePage methods need to run safety checks

MovePage methods need to run safety checks

A move method that doesn't check for things like moving a page on top of
itself or moving to a namespace with a different content model does not
seem like it's what callers would expect, and not what Title::moveTo
ever did. If there's a caller that really wants this behavior, we can
make moveUnsafe public.

I also made the $reason and $createRedirect parameters to move()
optional to match Title::moveTo() behavior. However, I made $reason
default to null instead of '', to distinguish between an empty edit
reason provided by the user and no edit reason provided at all (e.g., a
move done internally without specific user request).

Depends-On: I971e619eb76c4474fe037fad258f9c496717bf41
Change-Id: I6ddcc9f34a48f997ae39b79cd2df40dd2cc10197