Repository servlet can only find referenceable nodes by UUID

The repository servlet can only find nodes by UUID that have the mixin mix:referenceable. However, since JackRabbit 2 all nodes have a UUID, so it should be possible to find all nodes by identifier.

Steps to reproduce:
1. login to the console
2. lookup the UUID of the node /hippo:configuration
3. open the repository servlet (http://localhost:8080/cms/repository)
4. paste the UUID of /hippo:configuration in the 'UUID' field and click 'Fetch'

Expected: the node /hippo:configuration is found
Actual: no node is found