Add bureau page text & media object, content title/description, and content_block modifier patterns

- Add Bureau landing page text
- Add `content_block__padded-top`,  `content_block__border-top`, and `content_block__border-bottom` modifiers
- Add OOCSS-based `media` object
- Add `content_title` and `content_desc` hierarchy classes
- Add `list__branded` with CFPB-green bullets
- Add `u-show-on-mobile` and `u-hide-on-mobile` utility classes
I'm going to move the breakpoint labels discussion to the issues because it shouldn't hold up this PR. https://github.com/cfpb/cfgov-refresh/issues/149