"Source" tab is shown even when no source is present

[pietroalbini]

#519 fixed a panic happening when visiting a crate's source page when no source was available (due to a build failure), by showing a 404 page instead.

Still, the link to the source tab is unconditionally present in the navbar, even when we know it will lead to a 404 page. We should fix this by hiding the link in those cases.

[jyn514]

This is now much less of an issue due to #928 (since there will always be source present from now on). However there will still be 404s for crates built before that PR.