Bug: Validate `Key` on Rename

[JeremyCaney]

Currently, changing the Key to an invalid value throws an exception that’s unintuitive related to the rootTopic. At minimum, it should throw a InvalidKeyException. Better yet, it should return a validation error to the interface.

It also seems like it may be caching the invalid key in memory, causing subsequent issues. That needs to be verified, though, based on an informal customer report.

[JeremyCaney]

This was resolved by adding in an explicit check for TopicFactory.ValidateKey() in the Edit() action handler, and returning a model error so there's a friendly error returned to the client (e79a823).