Expand File List block at page level

[brynwhyman]

Replaced with generic issue to expand blocks: https://github.com/silverstripe/silverstripe-elemental-blocks/issues/43

User Story

As a CMS user, I want to be able to expand the file block preview to view the fields of the block, so that I can see what is available for editing.

Acceptance Criteria

• [ ] An expand arrow is displayed on the block preview.
• [ ] When I click on the entire block (incl. the down arrow) the mini preview disappears and the block expands to show its contents of editable fields.
• [ ] The arrow changes from down to up when the block is expanded and vise versa.
• [ ] The block can be collapsed again by clicking the up arrow. The editable fields will disappear and be replaced by the mini preview with any new changes if saved/published.
• [ ] File list block is displayed as per the wireframe when clicked: https://projects.invisionapp.com/share/ZRL4T2HQAGB#/screens/301923216
• [ ] If the block has content that is currently saved, this content is presented when the block is expanded.
• [ ] If the block has content that is currently published and no saved draft content, the published content is displayed when expanded.