Remove jQuery in the frontend for Twenty Twelve.

[adamsilverstein]

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

Refreshed version of #1683 (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 twentytwelve-navigation and the aria-expanded toggling on the menu button - are preserved in the vanilla JS implementation.

Trac ticket: https://core.trac.wordpress.org/ticket/54171

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[adamsilverstein]

Merged the latest trunk (clean, no conflicts) and fixed several runtime bugs found while re-reviewing the vanilla JS against the original review feedback on #1683:

• getSiblingElements() iterated parentElement.children (an HTMLCollection, which has no forEach) and the focus/blur listeners were attached via getElementsByTagName( 'a' ).forEach() - both threw a TypeError at load, breaking the whole script.
• The touchstart handler had lost its negation: it only acted when the item already had the focus class, so the first tap on a parent item navigated immediately instead of opening the submenu. Restored the original behavior (first tap opens, second tap navigates).
• Removed the duplicate per-link touchstart listeners in favor of the single delegated handler on body, which also covers dynamically added menu items (as discussed in the original review).
• Registered the delegated listener with { passive: false }, since browsers default to passive touch listeners on body and preventDefault() would otherwise be ignored.

Verified in a browser harness: menu toggle updates aria-expanded in both directions, focus/blur toggle the focus class on ancestor items, and the first tap on a parent item is prevented and opens the submenu while the second tap is allowed through. jshint passes.