defaults: add customizable defaultiTunesAppId

App URLs are already theme customizable however the itunes html meta app-id isn't yet.

Please review thoroughly before merging.
In the same source file, within 10 lines of code, print_unescaped is being used at least five times. So I was merely being consistent with existing code.

Presumably it makes more sense to leave my commit as-is, and refactor all six seperately?