Include apple-touch-icon in PWA icon requirements

[paulirish]

iOS PWAs are a thing! Right now mobile safari doesn't use the icons defined in the web manifest and instead uses the <link rel="apple-touch-icon" href="apple-touch-icon.png"> for the PWA homescreen icon.

We should include this in the PWA checks.

Firt said "The test should check probably presence, icon size and the absence of transparent colors on it 🙄"

webhint checks some of these aspects: https://webhint.io/docs/user-guide/hints/hint-apple-touch-icons/

thread with firt: https://twitter.com/paul_irish/status/1112791885246951424

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Related to icons, we could also check for <link rel="apple-touch-startup-image" href="/launch.png"> which is the image used in the splash screen. Unfortunately, the png asset has to be the same size as the device screen, which leads to a bunch of ugly tags. Let's hold off on this part until we get guidance from the Safari team.

Related: https://developers.google.com/web/updates/2018/07/pwacompat for all sorts of crossbrowser PWA compat

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Slightly related bug: #6700 for changing the icon size requirements