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

It's just a bit of fudging so the indicator doesn't wait to trigger the second the section hits the top of the page (since I don't think people usually read the lines exactly at the top of their browser window when scrolling).

Not scientific, but would be interested in more thought on that. Added a better note to the code.