Add 'featured books' section

[SteelWagstaff]

As a network manager, I want to showcase a few 'featured books' in a place of prominence on my catalog.

I should be able to customize the display text for this section (default: "Featured books:") and select up to 4 'featured books' for a network.

Possible implementation: 'customizer' option that lets them 'turn on' featured books and customize text. 4 drop down selectors that let me choose from any book in the network catalog as a featured. 'Featured book 1' 'Featured book 2', etc.

These options should replace the current 'Front page catalog' block.

See wireframe for design spec: https://www.figma.com/file/mQ7n8LlKjxWdUCdne4rTdk/Network-Catalogue-Redesign?node-id=0%3A1