Applicant count delete check
This MR resolves https://gitlab.developers.cam.ac.uk/uis/devops/digital-admissions/applicant-document-management-service/-/issues/52
Oops - looks like I also included a tiny bugfix to the validator (as a separate commit), hopefully this can be reviewed at the same time. There was some discussion that we might make all of the college drives actually folders within a single drive. The validator currently has a bug where it checks the location/name/metadata of the directory that it is pointed to (except where this directory is the top level drive). This fixes that bug.