Enables toggling of ACF Flexible Content Fields for easier sorting and management.  Inspired by "Advanced Custom Fields Repeater Collapser" by Mark Root-Wiley (https://github.com/mrwweb/ACF-Repeater-Collapser). Requires the "Advanced Custom Fields" plugin and the premium add-on "Advanced Custom Fields: Flexible Content Field" by Elliot Condon (http://www.advancedcustomfields.com/).