Customizer: Introduce WP_Customize_Control::active() method to determine whether the control is relevant to the current context (i.e. to the current URL being previewed).

Control can indicate its active state by a subclass overriding the 'active_callback' method, by supplying a callable 'active_callback' argument into the control's constructor, or by filtering 'customize_control_active'.

props westonruter.
see #27993.
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