Add effort-based rate limiting

[Noppes]

Currently, Fluffle does nothing when it comes to rate limiting because all-in-all it is quite efficient and the hareware it runs on is fairly decent...

However, processing animations and videos will consume way more resources than images do. At that point it will be beneficial to implement some rate limiting system.

The rate limiting system should be effort-based. Meaning the more resources a request takes up, the more effort it costs. This will also reward API consumers to interact with the API in an efficient way (resizing images, animations and videos before sending them over, for example).