Adds a deep link to a mobile app if so configured

Adds, if an App-ID is configured, a link to the respective
page in order to allow deep linking from the Web to a
mobile app, like the Wikipedia app, as per
https://developers.google.com/app-indexing/webmasters/details
If no App-ID is configured, no link will be created.

Bug: 60537
Change-Id: Icadc7ab1e050e1e3d28cbd9d41c1b35b42e3fcca
Merge "Adds a deep link to a mobile app if so configured"