Twenty Fifteen: Don't save unfiltered CSS in a setting.

Introduce `twentyfifteen_get_color_scheme_css( $colors )` which will be used for Underscore/Backbone and the PHP side. Let `twentyfifteen_color_scheme_css()` handle the color calculation on PHP side.

see #30409.
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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()`.

see #30724.
Built from https://develop.svn.wordpress.org/trunk@30896


git-svn-id: http://core.svn.wordpress.org/trunk@30886 1a063a9b-81f0-0310-95a4-ce76da25c4cd