Customizer: Export Preview instance to `wp.customize.preview`.

Merges [30891] to the 4.1 branch.

see #30409, #30726.

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Twenty Fifteen: Don't escape translated strings.

Replace every unnecessary instance of `esc_html__()` and `esc_html_e()` with just `__()` and `_e()`.

Merges [30896] to the 4.1 branch.

see #30724.

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Twenty Fifteen: Remove `esc_html()` from blog description.

The blog description gets esc_html()'d *into* the DB. It's also escaped because the filter for `get_bloginfo()` is set to 'display'.

Merges [30897] to the 4.1 branch.

see #30724.

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Twenty Fifteen: Remove various unnecessary `esc_html()` calls and replace various unnecessary `esc_html_x()` calls with `_x()`.

Merges [30899], [30901], and [30905] into 4.1.

Props ocean90
Fixes #30724

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