[Dashboard][Collapsable Panels] Lower bundle size

[ThomThomson]

kbn grid layout should be smaller than react grid layout in terms of bundle sizing. If it is larger, or if we'd like to lower the size even further we can optionally consider the following:

1. Make the kbn grid layout package take a new isEditable prop.
2. Make a simple fallback component that renders each panel as simply as possible via CSS grid with no drag events or any other code.
3. When the grid is rendered with isEditable false, it should render the simple fallback.
4. When the isEditable prop changes to true, the @kbn/grid-layout package can async import the editable version of the grid, continuing to show the non-editable component while the editable one loads.

In this way we can reduce the bundle size of our layout engine to near nothing (a simple CSS grid component).