"previous page" button never present in search results

[GuillaumeGomez]

It's apparently because crates.io seek pagination doesn't allow it (without sacrifing performance) so it's up to us to fix it.

We can still use the old pagination system by providing the page number. I don't really see another way to fix it sadly...

[syphar]

I lean towards our search just being a thin wrapper on top of crates.io search.

Which could mean:
don't show a prev-button when there is no prev-link.

The old pagination system is quite tough on the database, so I wouldn't revert to using it for crates.io's sake.