Wrap up bureau landing page

- Add javascript & css blocks to base layout to allow for page-specific code.
- Add `slick.js` carousel plugin
- Display bureau landing page `mission` media blocks as carousel on mobile-sized viewports.
- Add `MobileOnlyExpandable` functionality.
- Display bureau `functions` content in expandable on mobile-sized viewports.
- Add `BreakpointHandler` to register callbacks on viewport resize.
- Add `centered` modifier for media blocks.
- Remove `content__max-width` class.
- Add `expandable_header__jump-link` modifier to style expandable headers like jump links on mobile.

`@gray-50` is close enough, it's ok to stick to the color vars we have.