Add reordering products inside ranges
Fix PEP8
Clean range ordering

- Remove forgotten print statement
- Rename `RangeOrdering` to more generic `RangeProduct` (later we can
  add another fields that are not ordering specific)
- Use `display_order` rather than priority (we set the order using
  jQuery sortable, not bump priority of couple products)
Fix backwards incompatible changes
Merge pull request #1020

Conflicts:
	docs/source/releases/v0.6.rst