Disallow moving Schema pages

Renaming Schema pages introduces mutability to a design centered around
immutability, which opens up a can of worms. When a Schema is renamed, do we
continue accepting events logged under the previous name? Do we validate older
names, and if so, how? These questions don't admit simple, direct answers.
Disallowing moves will likely inconvenience schema authors and maintainers, but
the cost (measured in software complexity) of allowing moves is too high.

Change-Id: I996e88c35aea872ab49f8dea106c335758896132