Cache RIT Images better

This is a bad cache
https://github.com/ComputerScienceHouse/packet/blob/f61cd2b5b616e3b7f35fad95b515313ea2d50bcb/packet/context_processors.py#L58-L70

It caches the first call to this function for any given username (up to 256 of them) via [`functools.lru_cache`](https://docs.python.org/3/library/functools.html#functools.lru_cache), which is bad, because it kinda defeats the point of gravatar.

We _could_ do some form of caching, but probably shouldn't. Loading these images lazily in the frontend would probably be much better.

At the very least, please remove the cache so we can stop kicking the pods to have photos update.