Add Lookbook

[lorenyu]

Ticket

Resolves #93

Changes

• Add lookbook gem and related dependencies
• Add ENABLE_LOOKBOOK environment variable, defaulting to true in local and dev environment
• Mount lookbook if ENABLE_LOOKBOOK is set
• Add component_preview layout for previewing partials without application header/footer
• Add breadcrumb preview
• Make partial paths explicit

Context for reviewers

Add Lookbook. Took a while to get something working since Lookbook docs don't make it clear:

1. You cannot use partial:
2. You need to specify template: explicitly in render method (cannot just pass a string to render) which they claim is possible from the examples in their documentation
3. You need to use explicit paths to partials within templates

Testing

see navapbc/platform-test#205