Enhance order dashboard

- Add more links
- Add event details to line detail page
Rework shipping event validation
Introduce payment event subtotal calculation
Rework the shipping event testing methods
Document how order model can be extended to validate shipping events
Indentation!
Don't send 404 emails in demo site
Convert Address name and salutation to properties
Reorganise address tests
Add verification of payment events
Add migration for order address models

Picks up the postcode being NULLable in the address superclass.
Rename date field of event models to date_created

To be consistent with the rest of Oscar.
Further document EventHandler class
Add reference field to payment event

This is useful for linking between payment events and transactions.
Add FK to shipping event from payment event

This allows them to be linked together which is often how the occur in
the real world.
Avoid needless extra SQL queries when placing order
Improve display of order transactions
Shrink thumbnails to 350px
README improvements

- Add Freetix screenshot
- Copy reworking
- Drop link to Mirumee :(
Add community extensions to README