Nav menus: Allow assigning a new menu to an existing location when no theme setting is currently saved.

Merges [25195] to the 3.6 branch.

props ocean90.
fixes #25064.

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Remove display of 'Previously restored by' in the revisions meta box as it is busted.

Fixes invalid markup (missing closing ul tag) when there was no previously restored revision.

Merges [25194] to the 3.6 branch.
fixes #25051.


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Fix 'html5' theme support.
 * Merge, rather than replace, on second add.
 * Make current_theme_supports() work when two arguments are passed.
 * Require the second argument to be an array.

Merges [25193] and [25235] to the 3.6 branch.
fixes #24932.

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