Flux Refactor + SubItems UI

[wookiehangover]

What does it do?

• Moves most asynchronous server interactions out of the ProductStore and into ProductActions.
• Reduces dependency on sprintly-data collections directly in ItemColumn component
  • More work can be done here because there are still implied assumptions due to how filters work within an item column
• Adds a dedicated UI for subitems

This depends on upstream changes in sprintly-data https://github.com/sprintly/sprintly-data/pull/16

Where should the reviewer start?

This seems like a big changeset, but most of it is copying methods and tests between app/stores/product-store.js and app/actions/product-actions.js.

Subitems changes come nested within their parent items directly from sprintly-data, so the only changes we needed to account for were global settings for product API requests and additional comonents within ItemCard.

How should it be manually tested?

Head over to https://sprintly-manifold.herokuapp.com/product/1 and give it a test drive!

Background context

The reason for the flux refactor was to get our data flow to better align with the Flux pattern. Instead of relying on backbone events to trigger re-renders, our store now emits it's own events, and the store implementation has less knowledge of server interactions.

User Interaction --> ActionCreator calls API --> Action Dispatched --> Store emits change

Screenshots

[phoenixbox]

@wookiehangover seems good - nothing apart from nitpicks - there are some pending xdescribe tests also but :shruggie: