Add deleteGroup and deleteUser to IAdmin

[jamoore]

Extracted from #545.

Deleting groups and users is obviously tricky. Either it must be done recursively, or can be done simply but will only succeed if there are no FK links to the entity.

Most likely the simple delete will suffice. If so, then IUpate.deleteObject() could be used if BasicACLVoter is updated to allow deleting Experimenter and ExperimenterGroup (test before attempting).

[jmoore]

Will have to wait for the next round of delete improvements.

[jmoore]

See #1329

[jburel]

if we let users create their groups, such functionality needs to be considered using the delete service.

[jmoore]

Definitely, but I don't see us implementing and testing user/group deletion for 4.4.

[mtbcarroll]

A naïve look at this with my current experimental branch goes after the event log as if it were assigned to the Ministry of Truth, the most obvious initial problem being that parent directories in the repository may have an OriginalFile instance whose creationEvent has an experimenterGroup of the target group so it even goes after that instance and runs into PSQLException complaints of non-empty directories. Further work on this probably requires some initial scoping with such issues in mind: how to circumscribe this group deletion and how to adjust outstanding not-null properties that reference the target group across that border.