Limit share frequency

[cgay]

To avoid disk full DOS attacks, limit how often one client can share code. Guessing it has to be IP based.

[cgay]

I ameliorated this via the nginx rate limiting configs so I'm going to downgrade priority. It's still quite possible for a determined person to fill up the disk but it should now take them a lot more time.

TODO:

• application level detection that adds IPs to a deny list, at least until server restart. (git stash pop rate-limit)
• display a decent error message when the 503 response is received by the client.