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.

从 packet/context_processors.py 的第 58-70 行开始，检查 RIT 图像查找周围对 functools.lru_cache 的使用。删除缓存作为最小修复，然后验证无需重启 pods 即可显示更新后的图像；lazy frontend loading 是 issue 提到的一种可选替代方案，但不是必需的。

由索引模型根据 Issue 内容生成。