Clean up order detail template

- Don't show discounts headers if no rows
Improve order detail dashboard view

- Rework view to use simpler methods
- Rename reloading method (not sure why, for readability I think)
- Use a form for changing the order status (we should be using forms for
  changing line stuff too - will look at that next)
- Generally clean up the view a little
More cleaning of order detail template

- Rename offers to discounts
- Use tabular display for payment information (to be consistent with
  other tabs)
- Show customer's email address when they checked out using guest
  checkout
Further improve order detail view

- Push saving order note logic into form (and out of the view)
- Hide order status form when there are no new options to choose
Use a line status cascade pipeline in sandbox

So we can see cascading line status changes when an order's status is
changed.
Use dynamic loading for forms on order detail page
Improve functional tests for order detail page

This change includes some tidying and organisational changes. There's
also a new test for adding order notes.
Correct naming for delivery date, and minor style fixes

* AFAIK, Oscar doesn't have functionality for delivery dates. Changed
  the wording to be consistent with the field name and wording in other
  templates.
* Line length fixes
* Refactored imports
Update v0.8 release notes re order detail changes
Merge branch 'rework-order-detail-view'

Conflicts:
	docs/source/releases/v0.8.rst
Prune dead autodoc references