Add data retention policy

doc/design/data-retention-policy.md

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+# Data Retention Policy
+
+Dandihub data storage on AWS EFS is expensive, and we suppose that significant portions of the data
+currently stored are no longer used. Data migration is where the cost becomes extreme.
+
+## Persistent Data locations
+
+Each user has access to 2 locations: `/home/{user}` and `/shared/`
+
+(Within jupyterhub `/home/{user}` the user always sees `/home/jovyan`, but is stored in EFS as their GitHub
+username.
+
+## Known cache file cleanup 
+
+We should be able to safely remove the following:
+ - `/home/{user}/.cache`
+ - `nwb_cache` 
+ - Yarn Cache
+ - `__pycache__`
+ - pip cache
+
+
+## Determining Last Access
+
+EFS does not store metadata for the last access of the data. (Though they must track somehow to move
+to `Infrequent Access`)
+
+Alternatives: 
+ - use the [jupyterhub REST API](https://jupyterhub.readthedocs.io/en/stable/reference/rest-api.html#operation/get-users) check when user last used/logged in to hub.
+ - dandiarchive login information
+
+## Automated Data Audit
+
+At some interval (30 days with no login?):
+   - find files larger than 1GB and mtime > 30 (?) days -- get total size and count
+   - find _pycache_ and nwb-cache folders and pip cache and mtime > 30? days -- total sizes and list of them
+
+Notify user if:
+   - total du exceeds some threshold (e.g. 100G)
+   - total outdated caches size exceeds some threshold (e.g. 1G)
+   - prior notification was sent more than a week ago
+
+Notification information:
+   - large file list
+   - summarized data retention policy
+   - Notice number
+   - request to cleanup
+
+### Non-response cleanup 
+
+If a user has not logged in for 60 days (30 days initial + 30 days following audit), send a warning: 
+`In 10 days the following files will be cleaned up`
+
+If the user has not logged in for 60 days (30 initial + 30 after audit + 10 warning):
+`The following files were removed`
+
+Reset timer.