Disable "Bulk Delete" of UnitTestInfo objects

Issue #171 resolved
Randle Taylor created an issue

Django does not give a warning about database objects that will get cascade-deleted when using the bulk-delete option. This can lead to a situation where a user inadvertently deletes TestInstances without warning when deleting a UnitTestInfo.

(An argument could likely be made for disabling any type of deletion of UnitTestInfo objects).