Add iOS attribution page and universal link support

[reemhamz]

One-line summary

We want to add universal link support on our website so users using iOS devices would be able to open the app store to access our products, and we should be adding fallback pages in case users do not want to open the app store directly.

Testing

To test this work:

http://localhost:8000/en-US/firefox/browsers/mobile/get-ios/

Demo: https://www-demo4.allizom.org/en-US/firefox/browsers/mobile/get-ios/