Log deleted requests and reinstate delete function

Following a request from Brighton to re-enable the admin's "delete request" function, this PR implements a simple method of logging the deleted requests without providing any additional functionality to make use of the captured data (that sounds like a larger piece of work deserving of fuller discussion).

Deleted requests are removed from the requests table and are copied into a deleted_requests table along with the deletion date and the email address of the staff member responsible.

The original delete icons have been restored to the relevant admin views.

Possible issues - orphaned requests on WDTK :frowning: (although this would have been the case until recently anyway)
should use `current_staff_member` instead