R1:847b7977c125

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…