Disable the feature of downloading external schemas for unregistered namespaces
Currently, Metacat will download the schemas during the metadata object inserting if it finds the namespaces is not found in the local cache system. In order to use the same schemas to validate xml objects, we need to disable the feature.
If the schemas of the object can't be found locally, Metacat will reject the documents (throw an exception). However, we need to provide those features for users:
1. A clear message to specify that the schema is not registered and contact Metacat operators.
2. A new admin page for Metacat operators to register schemas with/without namespaces.