Tidy spacing around search box/add hover state

- Toggle checkbox filters don't have a hover state, which they should do as they are clickable.
- Bonus: add retina versions
- The padding around the searchbox (it is different to the padding around the other form elements...) has always driven me crazy so I've fixed it.
- Bonus: fix width on ios
Merge branch 'master' of github.com:alphagov/finder-frontend into fix-text-box-margins

Conflicts:
	app/assets/images/toggle-sprite.png
Merge pull request #35 from alphagov/fix-text-box-margins

Fix text box margins