Customize: Allow previewed menus to be customized

Fixes a bug where menu assignements couldn't be changed when previewing a theme.
Also removes an unnecessary call to menu mapping after a theme switch from the customizer and makes sure the locations option is always written, for good measure.

Props westonruter.
See #39692.

Built from https://develop.svn.wordpress.org/trunk@41995


git-svn-id: https://core.svn.wordpress.org/trunk@41829 1a063a9b-81f0-0310-95a4-ce76da25c4cd