new feature: user account deletion

[jlipps]

Should we allow user account deletion? This comes with potential complexity such as what to do with data added by deleted users. Suggestion: rather than deleting an account, simply rewrite all personal information as a 'ghost' user? Or mark their data as belonging to a global 'ghost' account?

[jlipps]

If taking the first strategy, could also disallow API retrieval of data for that user (return 404)