Congrats to our register_header_metabox function! Has reached 301 lines!

1. Should be broken down to 4 smaller functions based on purpose.

register_meta_box_header, register_meta_box_post, register_meta_box_open_graph, register_meta_box_campaign

2. All these new smaller functions and probably 'cmb2_after_form_do_js_validation' as well should be moved out into their own class.