Only add Schema.org metadata if not already added by another plugin

[westonruter]

Currently when theme support is enabled, amp_print_schemaorg_metadata() is called at the wp_head action. However, if another plugin is adding this Schema.org metadata then we should defer to the meta that it adds to prevent duplicates. So I think we should augment AMP_Theme_Support::ensure_required_markup() to check if the script is present after output-buffering, and if so, then add amp_get_schemaorg_metadata() to the head element.

Something similar should be done eventually for the app manifest.

For both Schema.org metadata and app manifest, core should eventually be doing these itself, and that will help facilitate themes and plugins coordinating on the data that is output.