Add delete_isilon_allocation utility function

[claire-peters]

Summary

• Adds delete_isilon_allocation(resource_url, path, force=False) to coldfront/plugins/isilon/utils.py
• Discovers and deletes all items associated with an allocation (quota, snapshot schedule, SMB share, NFS export, Lab/Everyone subdirectories, root directory), skipping gracefully any that don't exist
• Refuses to proceed if the quota reports non-zero physical usage
• Prints a full list of what will be deleted and requires the user to type "yes" to confirm
• path matches the Subdirectory allocation attribute (e.g. rc_labs/smithlab), lab name is derived from the last path component
• Intended for interactive shell use to clean up after a failed allocation creation

Test plan

• Call from Django shell against a test cluster with all items present — confirm preview output and deletion
• Call with some items absent (e.g. no snapshot schedule) — confirm absent items are skipped silently
• Call against a path with non-zero quota usage — confirm ValueError is raised before any deletion occurs
• Type anything other than "yes" at the prompt — confirm nothing is deleted