CrateDetails should not be optional

[jyn514]

The more I look at this, the more I think this Option is not correct. If it's None, that means that no crate exists for the (name, version) pair:

docs.rs/src/web/crate_details.rs

Line 107 in 6a53a71

return None;

So we should either unwrap() because we've guaranteed that this crate exists, or return a 404. I'm ok with making this a follow-up PR. The good news is this means the templates never have to handle the case when details is None.

Originally posted by @jyn514 in #879