Component: Agenda Accordion

[timmyc]

This issue covers creation of a new component(s) that can be used to render the accordion section seen in the Agenda block:

We may consider using @components/panel-body for this task, though with the custom counter in the header area, it might need to be a custom component.

Also <FoldableCard /> from Calypso is quite nice.

This issue covers creation of the Clickable header region, which will expand/display the child region. This does not cover the rendering of child items/content.

Props
Some props the component should likely support:

• title string
• isOpen bool
• count int