Recursive Deletes

[gbdubs]

• Deletes analysis based on a Portfolio if the portfolio gets deleted (satisfying the constraint we've implicitly encoded through the FKC's on the portfolio_snapshot's fields).
• Does the same for portfolio_group and initiative (though we can't yet run analyses based on these)
• Because this changes the signatures to return []pacta.BlobURI on deletes, adds blob deletion code to the API.

NOTE: I haven't been able to validate through running locally that the deletes for PGs or Initaitives work when this is run because we don't have the ability to run portfolio group or initaitive based analyeses yet.