Search results should be scrollable

[MaxGabriel]

When you press s on a haddock page, you get a search box to search the package. When you have many search results, the search box overflows the page. This has two issues:

1. You can't scroll the box, so you can't see all the search results. I have confirmed additional search results do exist by seeing there are additional DOM elements with more results
2. The box taking up the entire page is a little awkward

To repro:

1. Visit https://hackage.haskell.org/package/amazonka-1.6.1
2. Press s
3. Search "Auth" (case sensitive—auth doesn't have many results)

I think a good fix would be to add:

overflow: scroll;
max-height: 80%; 

to the #search-results div. Any max-height will do, not specifically 80%. I've tested this works by making edits in web inspector.