[Obsolete] Trac 58974: Modernize wp-embed script to remove IE≤11 code and support for WP<4.4

[westonruter]

• Remove obsolete load event handler in wp-embed since IE10+ support DOMContentLoaded.
• Replace obsolete use of document.createElement('a') in favor of the newer URL class (supported in all browsers but obsolete IE11).
• Remove obsolete IE10/IE11 code.
• Combine condtionals.
• Use substring() instead of deprecated substr() method.
• Eliminate the stipulation that wp-embed.js not include ampersands, considering this was put in place for WP<4.3 which now accounts for only 1.43% of sites. This includes the elimination of the verify:wp-embed grunt task.

Trac ticket: https://core.trac.wordpress.org/ticket/58974

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[westonruter]

I erroneously pushed the branch to origin and not to my fork. Closing this PR in favor of #4967

[westonruter]

I erroneously pushed the branch to origin and not to my fork.

This won't happen again, thanks to:

git remote set-url --push origin https://github.com/westonruter/wordpress-develop.git