Nominations: Clean up

[wookie184]

I'd like to:

• Remove the cache of nominations, as we refresh it almost every time before using it anyway so it's pretty useless. It's also a potential for bugs in places we forget to keep it updated.
• Convert the site response of nominations into dataclasses or pydantic models. This means we don't have to use dictionary accessing everywhere and makes my IDE happy because it knows the types. (I think pydantic would probably be easiest as it handles dictionary->model conversion automatically)
• Remove support for manually posting a review/manually specifying review contents in the voting channel. We rely on message contents being what the bot sent in a couple places so I think this could cause issues even currently. It's also not really necessary as you can still trigger votes manually and then comment any extra details in the thread.