server: validate stream names

[emilyalbini]

As suggested in #103 (comment), this adds some validation to stream names received by the server.

[jclulow]

I think factory_worker_append() in server/src/api/factory.rs also needs to use validate_stream()?

[emilyalbini]

Done both!

I think factory_worker_append() in server/src/api/factory.rs also needs to use validate_stream()?

I wonder, would it make sense to turn Stream on the server in a newtype Stream(String), and requiring validation when a new instance of Stream is created? That way we'd never forget validation wherever we decide to add events.