[Improvement] Allow bursting for rate limited requests

[jackellenberger]

For operations like downloading the initial emojilist, if you have more than 50 pages of emoji (and we do), then the minimum download time is SLACK_REQUEST_WINDOW = 60000ms. That's kind of a nasty user experience, esp because slack does allow bursting. We could get our 53 pages in no time at all by just running emojme with the SLACK_REQUEST_RATE set to 60, but that will override other rate limited endpoints to use that rate, which may fail in a sync type scenario.

If we just build in bursting, mayyyybe that'd make the problem easier. the implementation I imagine is just "go as fast as possible until we get rate limited, then abide by the limit."