Add method to delete user & resources (without re-assigning them)

[fbacall]

Summary of changes

• Adds User#purge to delete user + resources.

Motivation and context

Default deletion moves resources to the default user, which can be undesirable a times, for example when deleting spammers.

Checklist

• [x] I have read and followed the CONTRIBUTING guide.
• [x] I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree to license it to the TeSS codebase under the BSD license.