Item column data refactor

Moves column data management out of the column component and into the parent view (this has been needed for a long time, cc @florapdx)
Tracks column-by-column "loading" state in the product store, since it's the source of truth for all things columnar
Clears up flashes of "no results found" or "placeholder" cards during item load
Fixes a nasty bug where subitems were disappearing when added
Where should the reviewer start?
views/components/item-column/index.js
- move anything that was in state into props, since this comes from the parent now
- add a check for this.props.loading before trying to render anything
views/pages/items.js
- add another call to the _onChange handler to getItemsByStatus
- adjust how column components are created
stores/product-store.js
- added book keeping for column load status (that's reset on product init)
- added convenience method for getting all items by status for the column view
actions/product-actions.js
- remove a call to items.off() that was causing events to be lost in certain conditions
  Background context

Pulling the band aid off a bit here, but this definitely is a small step towards a cleaner data model.

giphy-2

sprintly / sprintly-kanban