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

https://bugzilla.mozilla.org/show_bug.cgi?id=1058292

Keeping all this obsolete code around was confusing, so I did a quick pass at removing it.

- `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- `header.js` has been replaced by `navbar.js`
- `collection` view/template/API endpoint is obsolete now that we have the feed.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.