Support executing CMS without installation, but from in directory different from its root.

Add a search path for isolate, which is built taking ../../isolate/isolate
with respect to the file that contains the module cms.grading.Sandbox.

Do the same also for the configuration file cms.conf.
Add a UniqueConstraint in Dataset on (id, task_id).

It's required, otherwise the ForeignKey fk_active_dataset_id
cannot be created.