Smarter handling of margins in two-wide embeddables

[#43056557]
Styles for optional two-wide or three-wide embeddables

[#43056557]
Logic to show embeddables two-across or three-across

For [#43056557] - this checks the count of embeddables and uses CSS classes for two-wide if it's an even number, and three wide if it's an odd number but doesn't leave a widow (e.g. 3, 5, 9, 11, but not 1, 7, or 13.)
Simplifying and "smartening" showing of custom MC options

For [#39917671], this ensures that the custom feedback form fields are shown when appropriate and toggle properly with the checkbox.
WIP on model scrolling and edit-mode full-width