iPhone/iPad install banner added.

What it does:
- Android Chrome keeps the native PWA install prompt.
- iPhone/iPad gets a custom premium banner with instructions:
  Share -> Add to Home Screen.
- It appears only on home pages.
- It does not appear if the site is already installed / standalone.
- Close behavior:
  1st close: may show again after about 1 minute
  2nd close: may show again after about 1 minute
  3rd close: pauses for 6 hours
- Added Apple PWA meta tags to home pages.
