[Bug 857148] Improve Rickshaw utils.

- Make k.rickshaw into a class, k.Graph.
- Merge most of the logic of the history chart into rickshaw_utils.js
    - Series colors
    - Choosing series sets
    - Slider controls
    - Timelines (annotations).
- Make rickshaw_utils.js's primary export a constructor instead of a
  simple function.
- Some graph controls are generated automatically, instead of done in
  the DOM.
- Feature toggling is done in JS instead of by presence of certain DOM
  nodes.