Operation slot machine

[zspencer]

@rosschapman ran into a weird bug when ActionTexting the MarkdownTextBlock where the action text couldn't save itself. Basically we were using nested attributes with a faux-relationship; which meant the actual MarktdownTextBlock was not getting saved.

@anaulin pointed out that this is an example of the single-table-inheritance being annoying, and that maybe we should finally deal with the cruft and blurry lines from our initial Furniture implementation.

So we started down the path of a refactor where we make a Slot responsible for where in a particular Section a Gizmo should live; and which Gizmo should live in that spot.

So we wrote a System Spec, and then we started implementing it and realized we had gone down the wrong pathyway, we want to create the Gizmo first and then add the Slot!

So we're going to reset and move forward with making a Card for each Gizmo!

• [ ] Change the Sections#edit page so that our new Gizmo management interface is a collection of CardComponents that link to the particular Gizmo's #new page.