Email notification acknowledging PrivacyRequest completed

[seanpreston]

Add the FIDESOPS__NOTIFICATIONS__SEND_REQUEST_COMPLETION_NOTIFICATION config var and default it to True.

If FIDESOPS__NOTIFICATIONS__SEND_REQUEST_COMPLETION_NOTIFICATION is True then send an email acknowledging that the data subject's privacy request has completed processing successfully.

If the request is an access request, it should include a link to the uploaded data package.

Questions

• what should the email copy read?
  • we should use placeholders for this during dev
• can we always provide a secure link via S3 only accessible for the user who has it?
  • we always have the URL but it's TBD on whether it's signed — so there could be complexity here
  • can we add restrictions to number of times available for download? — would they be settable from Fidesops' code or only at the bucket level?