Create data retention policy

[kabilar]

Points to discuss prior to creating policy.

1. Further document contribution of custom images to profile list.
2. Quotas for each user home directory. Either by providing message or hard cap.
3. Scratch directory for each user that is cleaned up after 30 days.
4. Shared directories for specific projects.
5. Alternative options for Datalad, since Datalad Dandisets creates many files for each user.
6. Remove cache directories for migrations (e.g. cache, nwb-cache, py-cache, yarn-cache).

[asmacdo]

Related https://github.com/dandi/dandi-hub/issues/177

[kabilar]

• [ ] Once the data retention policy is reviewed, we need to update the DANDI Terms and Policies as appropriate.
• [ ] If the changes to the DANDI Terms and Policies is significant, MIT General Counsel Office may need to review.