Lightweight publication burst reports

[LawnGnome]

Coming out of today's team meeting: it would be nice to have some sort of reporting of recent bursts of crate publication from a single account that are technically within the rate limit, but involve a sufficient number of crates that a crates.io team member should have a quick look at the account to make sure they're not violating the guidelines.

A cron job and a well placed query should get this done. I'm thinking nightly, with reporting on the previous 48 hours, and an e-mail sent to (insert address(es)/Zulip stream here) with anything worth checking.

(In the longer term, this is the kind of thing that could feed into a dashboard, or could be part of the scoring that goes into a quarantine system, but a cron job + e-mail feels like the right level of investment right now to me.)