Expand/collapse content blocks

Acceptance Criteria

1. When building or editing a document users can collapse the various 'content blocks', e.g. text, image/video/slideshow etc.

• users can expand collapsed blocks
• when collapsed some of the block is still shown (so the user can identify what block it is)

In the 14.0 restyling effort, the content block styling was changed, but a few things still need to be done:

1. Collapse and expand for compounds and content blocks
2. Change default of 'Show content block title' to true
3. Remove the the background colours of the compounds and content blocks
4. Change the padding of the last child in compound: it takes up too much space resulting in a lot of whitespace between field and compound border

Resulting 'look' (designs): 
https://bloomreach.invisionapp.com/d/main#/console/17292860/381968324/preview

https://bloomreach.invisionapp.com/d/main#/console/17292860/381968329/preview

Or:
https://bloomreach.invisionapp.com/share/Y8RKJJA5R3C#/screens/381968324

https://bloomreach.invisionapp.com/share/Y8RKJJA5R3C#/screens/381968329