Send submission notification as digests

[ctueck]

As discussed in our longer call the other day, submission notifications should be queued and sent as digests at regular intervals (I suggest to start with hourly or 2-hourly digests to keep it close to real-time, but we might tweak the interval later).

This should solve two problems:

1. If agencies make bulk uploads of dozens or hundreds of reports in separate API requests, there's an avalanche of emails sent.
2. If agencies upload files (rather than passing URLs) via API or webform, they get a submission email with a flag/warning. This is because the email is sent already before the request to the file submission endpoint is made, even if that's only seconds later. If the email goes out only after a while, the flag will have usually cleared by then.

In addition, we discussed that the submission report/log should be sent as a CSV file attachment rather than in the body of the email.

(#208 is related to that, but separate. #136 is about the content of the response.)