Updated checkbox control code

Previously, the checkbox control was built like a radio control,
meaning the option choices had to be given their own values based on
what each option represented. However, this doesn’t need to be done for
checkboxes. Checkboxes are true boolean. It’s either checked or it’s
not.

Checked values are always saved as 1. Unchecked values are always saved
as 0. Therefore, there’s no need to set a “choices” key in the
add_control() method with different values for checked and unchecked. Because "type" is set to checkbox, the system already understands the possible choices... 1 and 0.

Across the theme, all references to “option1” (“option2” was never
used) were changed to 1. Also, the add_control() method was updated to
reflect this change. Radio controls are not built into the customizer
by default so the add_control() method has to extend the
WP_Customize_Control class for it to work. That’s not the case with
checkbox controls because they are built into the customizer by
default. So the class extension was reversed and the add_control()
method for the checkbox was taken back to default.