Reorganization

# Changes
 - Organizes patterns into components, modules, and regions.
 - Improved scroll indicator performance (throttled; more efficient finding which element to highlight).
 - Removed unused `.embed-container` pattern (duplicate of `.media-video`).

For review: @DoSomething/front-end

:+1: `tile`, `polaroid`.

:information_desk_person: `message-callout`, `media-options`

:-1: `pullquote`