Add semi-generic rate limiting infrastructure

Summary:
This adds a system which basically keeps a record of recent actions, who took them, and how many "points" they were worth, like:

epriestley email.add 1 1233989813
epriestley email.add 1 1234298239
epriestley email.add 1 1238293981

We can use this to rate-limit actions by examining how many actions the user has taken in the past hour (i.e., their total score) and comparing that to an allowed limit.

One major thing I want to use this for is to limit the amount of error email we'll send to an email address. A big concern I have with sending more error email is that we'll end up in loops. We have some protections against this in headers already, but hard-limiting the system so it won't send more than a few errors to a particular address per hour should provide a reasonable secondary layer of protection.

This use case (where the "actor" needs to be an email address) is why the table uses strings + hashes instead of PHIDs. For external users, it might be appropriate to rate limit by cookies or IPs, too.

To prove it works, I rate limited adding email addresses. This is a very, very low-risk security thing where a user with an account can enumerate addresses (by checking if they get an error) and sort of spam/annoy people (by adding their address over and over again). Limiting them to 6 actions / hour should satisfy all real users while preventing these behaviors.

Test Plan:
This dialog is uggos but I'll fix that in a sec:

{F137406}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8683