Remove jQuery in the frontend for Twenty Fifteen.

[adamsilverstein]

• Replace jQuery usage in Twenty Fifteen frontend scripts with vanilla JS.
• Don't require jQuery to be loaded in the frontend as a result of the above.

Refreshed version of #1682 (the original branch lives on a fork that can no longer be updated), merged with current trunk. Trunk changes since the original PR - the deferred loading strategy for twentyfifteen-script and the aria-expanded accessibility improvements in the menu toggles - are preserved in the vanilla JS implementation.

Trac ticket: https://core.trac.wordpress.org/ticket/54172

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[adamsilverstein]

Merged the latest trunk (clean, no conflicts) and fixed two runtime bugs found while re-reviewing the vanilla JS against the original review feedback on #1682:

• The secondary toggle click handler compared classList.contains() (a boolean) against -1, making the condition always true, so aria-expanded was set to 'true' even when closing the sidebar.
• sidebar.getElementsByTagName( 'button' ).forEach() threw a TypeError because HTMLCollection has no forEach; switched to querySelectorAll() as suggested in the original review.

Also added null guards in onResizeARIA() / resizeAndScroll() to preserve the null-tolerance the previous jQuery code provided in case a child theme removes those elements.

Verified in a browser harness: dropdown toggles update aria-expanded/aria-controls and submenu classes in both directions, the sidebar toggle now correctly reports aria-expanded="false" when closed, and scroll/resize handlers run without errors. jshint passes.