Search - Hide yanked and outdated crates by default

[Mazwak]

When searching crates.io by relevance, some results are not (relevant) at all.

For example, if you search 'xlsx', the first page has :

• 2 yanked crates
• 5 outdated crates (> 2 years with no updates), 3 without even a README
• 3 'OK' crates

I don’t see any reason to return yanked crates (by default).

And a max age would be nice, maybe higher than 2 years though.

[carols10cents]

See #145, this is definitely something we want that someone needs to design and implement!