Decide between schema immutability and some locking concept

[alexanderdean]

Basic idea: once a schema goes live it should be locked, so it can't be overwritten.

Or maybe we should just have a rule that schemas can never be overwritten. That would be cleaner.

[alexanderdean]

Decision with @BenFradet : allowing override goes against versioning. Let's make them immutable for now, we can always invent some kind of override behaviour later if required. Closing.