Opening a crates versionless url links to prerelease versions

[Nemo157]

I just pushed a test crate to https://docs.rs/nemo157, there was a 0.3.0 version published previously and I just added a 0.3.1-pre version. I expected the versionless url to still redirect to the latest non-prerelease version, but instead it links to the prerelease version; and explicitly going to https://docs.rs/nemo157/0.3.0/nemo157/ shows a bright orange "Go to latest version" link.

I think for the purposes of promoting a stable ecosystem it would be best to treat prerelease versions as ineligible to be the latest version unless a crate has no normal versions available. Having them still listed under the versions for the crate and allowing them to be linked to directly would still be useful.

[udoprog]

I'd like to boost the signal on this a bit. It's causing a bit of a yanking party in tokio-rs/tokio#2036!

[jyn514]

@Nemo157 the test crate makes this much faster to debug, thanks :)

[jyn514]

I'm going to try and also fix #221 at the same time.

[jyn514]

This is mostly fixed, I'm just trying to solve #395 at the same time. If I don't get it in the next half hour or so I'll just make a PR without.