Fix redirects for apple-touch icons

We were redirecting to non-existent assets for /apple-touch-icon.png and
/apple-touch-icon-precomposed.png.  This started to fail noisily since
alphagov/static#289 was merged.

Add an override to redirect them to an asset that does exist. Also added
tests to ensure that the targets exist.
Ignore querystring when redirecting icons
Merge pull request #291 from alphagov/fix_apple_icon_redirects

 Fix redirects for apple-touch icons