Fix for search input in iOS

Without a webkit-appearence: none and border-radius: 0, iOS was
rendering the input box as a rectangle with circular ends.

I've also removed the `::-webkit-search-cancel-button` selector as
adding the one for the input will remove the webkit-appearence for the
cancel button too.
Merge pull request #687 from alphagov/fix-search-input-in-ios

Fix for search input in iOS8