Remove code no longer relevant because of the feed (bug 1058292)

- 'categories' API endpoint is useless
- /search/featured/ returns rocketfuel collections, which are going away
- header.js has been replaced by navbar.js
- 'collection' view/template/API endpoint is obsolete now that we have the feed.
- 'featured' view/template as well.
Implement device filtering UI (bug 969244)