refs #6949
- Adds a new VersionMismatchError with status 400 (bad request)
- Adds middleware that checks the X-Ghost-Version header if it is provided
- If it is not provided, the middleware does nothing
- If it is provided, and the versions match, the middleware does nothing
- If it is provided, and the versions don't match, the middleware returns a VersionMismatchError
- Includes both unit and a functional test to prove the middleware works alone and as part of the whole system
