Fix inject_ia_markup_meta_tag() to use is_singular() and get_queried_object_id()

[jerclarke]

re: #953

Small fix to ensure that before inserting the ia:markup_url meta tag, we check that we're on a single-post screen (is_singular() which is true for posts, pages and CPT single.php views).
Also uses with get_post(get_queried_object_id()) rather than relying on globals (no argument or using get_the_ID()) so that we always have the right post.

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